Revise policy, to allow init and system_server to configure,
clear, and read kernel trace events. This will enable us to
debug certain WiFi failures.

Note that system_server is restricted to only accessing
a wifi-specific trace instance. (Hence, system_server is
not allowed to interfere with atrace.) Moreover, even for
the wifi trace instance, system_server is granted limited
permissions. (system_server can not, e.g., change which
events are traced.)

Note also that init and system_server are only granted these
powers on userdebug or eng builds.

The init.te and system_server.te changes resolve the
following denials:

// Denials when wifi-events.rc configures tracing
{ write } for pid=1 comm="init" name="instances" dev="debugfs" ino=755 scontext=u:r:init:s0 tcontext=u:object_r:debugfs_tracing_instances:s0 tclass=dir permissive=1
{ add_name } for pid=1 comm="init" name="wifi" scontext=u:r:init:s0 tcontext=u:object_r:debugfs_tracing_instances:s0 tclass=dir permissive=1
{ create } for pid=1 comm="init" name="wifi" scontext=u:r:init:s0 tcontext=u:object_r:debugfs_tracing_instances:s0 tclass=dir permissive=1
{ write } for pid=1 comm="init" name="tracing_on" dev="debugfs" ino=18067 scontext=u:r:init:s0 tcontext=u:object_r:debugfs_wifi_tracing:s0 tclass=file permissive=1
{ write } for pid=1 comm="init" name="buffer_size_kb" dev="debugfs" ino=18061 scontext=u:r:init:s0 tcontext=u:object_r:debugfs_tracing_instances:s0 tclass=file permissive=1

// Denials when system_server sets up fail-safe
// (auto-terminate tracing if system_server dies)
{ search } for pid=882 comm="system_server" name="instances" dev="debugfs" ino=755 scontext=u:r:system_server:s0 tcontext=u:object_r:debugfs_tracing_instances:s0 tclass=dir permissive=1
{ read } for pid=882 comm="system_server" name="free_buffer" dev="debugfs" ino=18063 scontext=u:r:system_server:s0 tcontext=u:object_r:debugfs_wifi_tracing:s0 tclass=file permissive=1
{ open } for pid=882 comm="system_server" path="/sys/kernel/debug/tracing/instances/wifi/free_buffer" dev="debugfs" ino=18063 scontext=u:r:system_server:s0 tcontext=u:object_r:debugfs_wifi_tracing:s0 tclass=file permissive=1
{ getattr } for pid=882 comm="system_server" path="/sys/kernel/debug/tracing/instances/wifi/free_buffer" dev="debugfs" ino=18063 scontext=u:r:system_server:s0 tcontext=u:object_r:debugfs_wifi_tracing:s0 tclass=file permissive=1

// Denials when system_server toggles tracing on or off
// (WifiStateMachine is a thread in system_server)
{ search } for pid=989 comm="WifiStateMachin" name="instances" dev="debugfs" ino=755 scontext=u:r:system_server:s0 tcontext=u:object_r:debugfs_tracing_instances:s0 tclass=dir permissive=1
{ write } for pid=989 comm="WifiStateMachin" name="tracing_on" dev="debugfs" ino=18067 scontext=u:r:system_server:s0 tcontext=u:object_r:debugfs_wifi_tracing:s0 tclass=file permissive=1
{ open } for pid=989 comm="WifiStateMachin" path="/sys/kernel/debug/tracing/instances/wifi/tracing_on" dev="debugfs" ino=18067 scontext=u:r:system_server:s0 tcontext=u:object_r:debugfs_wifi_tracing:s0 tclass=file permissive=1
{ getattr } for pid=989 comm="WifiStateMachin" path="/sys/kernel/debug/tracing/instances/wifi/tracing_on" dev="debugfs" ino=18067 scontext=u:r:system_server:s0 tcontext=u:object_r:debugfs_wifi_tracing:s0 tclass=file permissive=1
{ write } for pid=989 comm="WifiStateMachin" name="tracing_on" dev="debugfs" ino=18067 scontext=u:r:system_server:s0 tcontext=u:object_r:debugfs_wifi_tracing:s0 tclass=file permissive=1
{ open } for pid=989 comm="WifiStateMachin" path="/sys/kernel/debug/tracing/instances/wifi/tracing_on" dev="debugfs" ino=18067 scontext=u:r:system_server:s0 tcontext=u:object_r:debugfs_wifi_tracing:s0 tclass=file permissive=1
{ getattr } for pid=989 comm="WifiStateMachin" path="/sys/kernel/debug/tracing/instances/wifi/tracing_on" dev="debugfs" ino=18067 scontext=u:r:system_server:s0 tcontext=u:object_r:debugfs_wifi_tracing:s0 tclass=file permissive=1

// Denials when system_server reads the event trace
// (This happens in response to a dumpsys request)
{ search } for pid=3537 comm="Binder:882_B" name="instances" dev="debugfs" ino=755 scontext=u:r:system_server:s0 tcontext=u:object_r:debugfs_tracing_instances:s0 tclass=dir permissive=1
{ read } for pid=3537 comm="Binder:882_B" name="trace" dev="debugfs" ino=18059 scontext=u:r:system_server:s0 tcontext=u:object_r:debugfs_wifi_tracing:s0 tclass=file permissive=1
{ open } for pid=3537 comm="Binder:882_B" path="/sys/kernel/debug/tracing/instances/wifi/trace" dev="debugfs" ino=18059 scontext=u:r:system_server:s0 tcontext=u:object_r:debugfs_wifi_tracing:s0 tclass=file permissive=1
{ getattr } for pid=3537 comm="Binder:882_B" path="/sys/kernel/debug/tracing/instances/wifi/trace" dev="debugfs" ino=18059 scontext=u:r:system_server:s0 tcontext=u:object_r:debugfs_wifi_tracing:s0 tclass=file permissive=1
{ write } for pid=3537 comm="Binder:882_B" name="trace" dev="debugfs" ino=18059 scontext=u:r:system_server:s0 tcontext=u:object_r:debugfs_wifi_tracing:s0 tclass=file permissive=1

Bug: 27254565
Test: manual
Manual test:
- Build this CL along with CL:322337
- Verify that system boots, and that we can connect to GoogleGuest.
  (Testing of actual trace functionality with require some more
  patches in frameworks/opt/net/wifi.)
$ adb root && adb shell dmesg | egrep 'avc: denied.+debugfs'

Change-Id: Ib6eb4116549277f85bd510d25fb30200f1752f4d