Update audio processing API docs.

Updating the audio processing API docs to mention the fact that the app
must indicate in their InCallService and CallScreeningService service
entries that the service type is for "microphone" access, otherwise
background audio access will not be permitted.

Test: Docs only change.
Bug: 156042528
Change-Id: I3576dba445497791e6fa4e0a2fa42cf10eb1df5b
2 files changed