AudioFlinger: fix fast capture silence

Silencing fast catpure tracks was not properly implemented.
Fix by zeroing fast capture thread HAL read buffer if the silenced
track is the only active, or by invalidating the silenced track
if other non fast tracks are active.

Bug: 157708122
Test: repro steps in the bug
Change-Id: I88b19540815bc1491b9e76c1ae6b6f85e32afa8f
Merged-In: I88b19540815bc1491b9e76c1ae6b6f85e32afa8f
(cherry picked from commit 33403f0e)