Check if calling uid is system uid for setAllowedPolicyCapture.

AudioSystem.setAllowedPolicyCapture is only called from audio service.
In that case, checking if the calling uid is system uid to before taking
action.

Bug: 145115448
Test: dumpsys audio, query active playback configuration
Change-Id: I614ac881ecde6bad13f22f1382de701f314a7ada
Merged-In: I614ac881ecde6bad13f22f1382de701f314a7ada
(cherry picked from commit 140a34fb)