Make sure MainContentCaptureSession's control behaviour is not changed

Before moving to the background thread, most of the function in
MainContentCaptureSession is either run blokcing on the current thread
or posting to the main thread. Although we've done some optimization to
prevent unnecessary posting. The behaviour should only be presetned in
the treatment arm to ensure the experiment is not bias.

Test: CtsContentCaptureServiceTestCases, MainContentCaptureSessionTest.
BUG: 309411951
Change-Id: I8df106be6d9bcbeeaf5d236f0ed4c5fffd05b619