aaudio: allow start when flushing

The state switch in the start() code did not allow
a start when in AAUDIO_STREAM_STATE_FLUSHING.

The bug was a regression in R and was not in Q.

Bug: 158603375
Bug: 159900922
Test: test_various.cpp
Test: atest CtsNativeMediaAAudioTestCases
Change-Id: I742a9d3d07d445c0f3a80187b5a32cda5c700e82