aaudio: reset volume ramps to avoid start glitch

When started, the volume scaler was ramping down from
the level it was at when stopped.
This could cause a short noise.

The fix is to reset the flowgraph so that the ramps
start at their target value instead of ramping to them.

Note that this CL only updates the playback side
so that it can be more easily backported.

Bug: 296270351
Bug: 297132022
Test: use repro steps at b/296270351#comment14
Change-Id: I307b76972d56b49e338b1a07b3ed921e996f2050