AudioPolicy: do not cap a11y volume during call screening

  Add accessibility playback as exemption for volume capping
during call screening.

Bug: 133879660
Test: enable Talkback, receive a call, screen it, touch responses icons,
verify labels are read

Change-Id: I661a544261ef57e439b3eb8cf3d1dde1c31ac303
Merged-In: I661a544261ef57e439b3eb8cf3d1dde1c31ac303