sepolicy: add policy for mediacodec service

During video playback, mediacodec process
requests perf to enable certain optimisations.
When the call is made to perf library, below AVC
denial was coming:

SELinux : avc:  denied  { find } for interface=
vendor.qti.hardware.perf::IPerf pid=1236 scontext=
u:r:mediacodec:s0 tcontext=u:object_r:
hal_perf_hwservice:s0 tclass=hwservice_manager

to address it, add policy for mediacodec service.

Change-Id: I3b7170eb6369f759ffe131acbfa5596c802aa2a5