Fix camera usage tracking.

When a Camera object is finalized, it notifies the torch service that
it's no longer using the camera. This call used to not check whether the
caller actually is still using the camera and called release() before,
so depending on timing of the finalizer, it could happen that e.g. the
camera app released the camera user record of the torch app, leading to
the torch usage not being detected on next usage of the camera app.

Fix that by checking whether the caller of onCameraClosed() actually
owned the camera prior to releasing the record.
Also fix synchronization between accesses.

Change-Id: Ic01cbc3c89ab8d855e4613bfa1b2f548de919b73