Fix double call to TTS connection disconnect() on reconnect

- Sets the service connection to null when unbindService is called,
instead of in onServiceDisconnected. This avoids a double disconnect
if a call to onServiceConnected is received before a call to
onServiceDisconnected.

- Extended synchronize on runAction error handling and reconnection.
This prevents from reconnecting N times if N>1 threads enter this method
while there's issue with TTS service.

Bug:6993880
Change-Id: I5a387622c6032a18d17fc072029ae6be1a9b8e6c