property_contexts

On eng and userdebug builds (only), allow system server
to change the value of log.tag.WifiHAL. WifiStateMachine
will set this property to 'D' by default. If/when a user
enables "Developer options -> Enable Wi-Fi Verbose Logging",
WifiStateMachine change log.tag.WifiHAL to 'V'.

BUG=27857554
TEST=manual (see below)

Test detail
1. on user build:
   $ adb shell setprop log.tag.WifiHAL V
   $ adb shell getprop log.tag.WifiHAL
   <blank line>
   $ adb bugreport | grep log.tag.WifiHAL
   <11>[  141.918517] init: avc:  denied  { set } for property=log.tag.WifiHAL pid=4583 uid=2000 gid=2000 scontext=u:r:shell:s0 tcontext=u:object_r:wifi_log_prop:s0 tclass=property_service permissive=0
   <11>[  141.918566] init: sys_prop: permission denied uid:2000  name:log.tag.WifiHAL
2. on userdebug build:
   $ adb shell getprop log.tag.WifiHAL
   $ <blank line>
   $ adb shell setprop log.tag.WifiHAL V
   $ adb shell getprop log.tag.WifiHAL
   V
3. on userdebug build with modified WifiStateMachine:
   $ adb shell getprop log.tag.WifiHAL
   D

Change-Id: I9cdd52a2b47a3dd1065262ea8c329130b7b044db