MediaSession: Use Handler for sending a pending intent

When PendingIntent.send() is called with a null handler,
onSendFinished() will be called inside of the send() method.
Therefore in this case, if a thread holds a lock before
calling send(), and onSendFinished() also requires the lock,
a deadlock could happen.

Bug: 25653801
Change-Id: I71d9f708fc3d52b7b5713426ad6140cbf6e336ef