Fix testCameraPreviewMemoryUsage test.

Looper.quit() is asynchronous. The looper may still has some
preview callbacks in the queue after quit is called. The preview
callback still uses the camera object (setHasPreviewCallback).
After camera is released, RuntimeException will be thrown from
the method. So we need to join the looper thread here.

This change also fixes a potential race condition.

bug:2521202
Change-Id: If69bbb3125d1d30192563559579b87c20fa5aac8