Add AudioTrackTest#OffloadCompletion to audiotrack_tests

This test executes scenario of the offloaded playback
and verifies that the client receives "on stream end"
callback after drain completes.

Currently only APE compressed format is used, so the
test is ignored on most devices except CVD. Will add
other formats later.

Fixed up StreamHalAidl::drain to avoid returning error
when called multiple times by the client.

Bug: 373872271
Bug: 384431822
Test: atest audiotrack_tests
Change-Id: Idfbdf02bbda0e8823eaa76348767d7c99e2322bd