aaudio: handle shared disconnect before start

Unplugging or plugging in a headset while an AAudio stream
is open causes a DISCONNECT. If a disconnect occurs between
open and start on a shared MMAP stream then it leaves an orphan
MMAP port.

This resource leak can prevent MMAP from working until the
device is rebooted.

This was caused by the mMapStream getting deleted
and preventing stopClient() from working.

The fix involves using a mutex to protect mMapStream.

Also we check to see if the endpoint was disconnected
during startClient() and stop the client port if so.

Bug: 344011310
Test: see bug for repro steps
Change-Id: I7aadf1f2dbc7886deb2cb59faf72598614ad4258