Camera: Pause callback during stopPreview()

Camera2Client::stopPreview() returns when Camera3Device is drained
(i.e., all results are returned). However since results are
sent/received asynchronously through Camera3Device::getNextResult(),
Camera2Client could potentially receive additional frames after the
return of Camera2Client::stopPreview().

This CL fixes this by adding unpauseCallback() and pauseCallback() to
CallbackProcessor which allows us to explicitly pause and unpause
callbacks during stopPreview() and startPreview().

Bug: 177493760
Test: run cts -m CtsCameraTestCases -t android.hardware.cts.CameraTest
Change-Id: I5c13f50305b1a56f97d4e9c665b62959b3095d0e