Audio: Ensure input stream gets closed on the audio HAL server side

As IStream::close only releases internal resources of the stream,
deferring actual stream closing to IStream server object
destruction, which is delivered to the audio HAL server
asynchronously, there is a possibility for a race condition
when streams gets opened and closed in a tight loop as in the VTS
test.

Work around this problem by flushing RPC messages between
the client and the server, and inserting a delay before opening
a new stream.

Bug: 139329877
Test: VtsHalAudioV5_0TargetTest#AudioPrimaryHidlTest.GetMicrophonesTest
Change-Id: Id8744f6f21fd3bfa607f489364925eccbab17b5e