This makes it clearer that the C++ SurfaceTextureHelper owns its associated java object it.
In addition, arrange so that the SurfaceTextureHelper.stopListening
method (in java) can be called from any thread.
BUG=
Review-Url: https://codereview.webrtc.org/1988043002
Cr-Commit-Position: refs/heads/master@{#12941}