Fix external camera HAL crashes

External USB camera hal service crashes when disconnecting the camera.
ExternalCameraDeviceSession's desctructor is called and deletes mLock,
but mBufferRequestThread is still running and waiting on the same
mLock, which causes the "pthread_mutex_lock called on a destroyed
mutex" error.

Fix the issue by stopping the thread in destructor. Also refactors the
functions and remove unused conditions.

Bug: 289741662
Test: Test with USB camera
Change-Id: I06b1ff6e192a4bca16822785d65d68a6aae53414