audioflinger: implement silenced capture for mmap

Implement silencing of audio capture for idle UIDs for
MMAP streams.

Use audio HAL setMicGain() API to silence the mmap buffer for
exclusive mode and when only one client is active in shared mode.

Add new MmapStreamCallback method onStreamSilenced() for AAudio
service to selectively silence streams from silenced UIDs in
shared mode.

Bug: 72134552
Test: manual
Change-Id: I7c92ba0002bc5ba003c1a3c887edddf9ae53b538