av: stop puller before releasing encoder

When encoder is released, it will no longer turn to media codec source
for fill-this-buffer. Hence, the buffer queue within puller will not be
cleared by encoder.

Stop mPuller before releasing encoder to avoid being stucked in
AudioSource::waitOutstandingEncodingFrames_l() if audiosource reset() is
invoked from SFRecorder destructor.

Bug: 123065628
Change-Id: I78ecb2207ae595784204bd6392311dc194af306d
Merged-In: I78ecb2207ae595784204bd6392311dc194af306d