aaudio: fix some state transitions

Now requestStop() and requestPause() do the appropriate thing
based on state.
Flush is allowed when OPEN, PAUSED or STOPPED because there might
be pre-roll data.

Bug: 69810494
Test: test_various.cpp
Change-Id: Ie1b306b17734a58fa71b1742bb186482893656b7
12 files changed