Move CameraManager interactions to background

Move all interactions with the CameraManager to a background thread.
This makes initialization async, but it's ok as we will be initialized
to the correct state (and that will be propagated) once we register the
TorchCallback.

For now, we call init in the constructor, but a refactor will happen to
move this to a better call point.

Also, remove optimistically setting the state of mFlashlightEnabled when
setting it. Instead, make sure that we always reflect the device state
by only changing it from the callback.

Test: manual
Test: atest FlashlightControllerImpl
Fixes: 233870137
Change-Id: I9100c9101429f2dc840c6708ad3115a4eab6ab4b