audioflinger: Fix audio for WifiDisplay

AudioFlinger is not able to determine the correct
pid/tid for WifiDisplay and thus we do not pass checks
for CAPTURE_AUDIO_OUTPUT and RECORD_AUDIO permissions.

To fix audio for WifiDisplay, it should be safe to
always allow a trusted calling uid (AID_MEDIA which
has the same perms as AID_AUDIOSERVER).

Change-Id: Ifa46d8e77a43027645cad02a04263b58e134c3ad