fix(audio): clear audioSwitch synchronously in AudioSwitchHandler.stop() (#967)

Audio output could get stuck on the earpiece (very low volume) after a
disconnect() + connect() on a reused Room on Android 12+
(CommDeviceAudioSwitch).

stop() nulled `audioSwitch` inside the posted teardown runnable while
tearing down handler/thread synchronously. Because the field was not
volatile and the write happened off the lock on the handler thread, a
fast subsequent start() could read a stale (already-stopped) switch and
skip re-creation via the `if (audioSwitch == null)` guard. With no new
switch created, activate() never re-ran, so the routing cleared by the
prior deactivate()'s clearCommunicationDevice() was never re-asserted and
playout stayed on the earpiece until the next connect.

Clear `audioSwitch` synchronously under the lock and mark it @Volatile so
start() reliably observes the teardown and re-creates the switch. The
switch's stop() is still posted to its handler thread, since
AbstractAudioSwitch is not threadsafe.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>