AudioFlinger: more duplicating mixer fixes

Additional fixes to support buffer durations of less than
10ms on the Bluetooth software encoder path:
- Fix ClientProxy implementation to allow for less than
10ms timeouts in obtainBuffer()
- Dynamically adjust the wait time in OutputTrack::write()
to skip waiting if one of the output tracks is near underruning

Bug: 412835975
Test: repor steps in bug
Flag: EXEMPT bug fix
Change-Id: I82a471504258a4bff9cd10df1bae18515c2bb746