audio: add command to purge permission cache

Add a media_audio_policy service command and helper function
in ServiceUtilities to purge permission cache.

This permists to restore the use of permission cache for
CAPTURE_AUDIO_HOTWORD permission while still  allowing CTS tests to
adopt shell permission.

Bug: 185972521
Test: atest AudioRecordSharedAudioTest
Change-Id: Ib8963053d6d7734e474174f572762abe1a4be13b