service_contexts

Add the necessary permissions for |wpa_supplicant| to expose a binder
interface. This binder interface will be used by the newly added
|wificond| service (and potentially system_server).
|wpa_supplicant| also needs to invoke binder callbacks on |wificond|.

Changes in the CL:
1. Allow |wpa_supplicant| to register binder service.
2. Allow |wpa_supplicant| to invoke binder calls on |wificond|.
3. Allow |wificond| to invoke binder calls on |wpa_supplicant|

Denials:
06-30 08:14:42.788   400   400 E SELinux : avc:  denied  { add } for
service=wpa_supplicant pid=20756 uid=1010 scontext=u:r:wpa:s0
tcontext=u:object_r:default_android_service:s0 tclass=service_manager
permissive=1

BUG:29877467
TEST: Compiled and ensured that the selinux denials are no longer
present in logs.
TEST: Ran integration test to find the service.

Change-Id: Ib78d8e820fc81b2c3d9260e1c877c5faa9f1f662