camera: Make stop_session a blocking call

For stop_session, we want to wait for it to complete before return.
Otherwise, we run into race condition when next start_session
happens.

Bug: 11045420
Change-Id: I283f6e374898120e0bfb95d39fb9561809779498
Signed-off-by: Shuzhen Wang <shuzhenw@codeaurora.org>