CameraManager: Handle camera service being disabled explicitly

Previously, CameraManager handled a disabled camera service
implicitly, the same as it handles a temporarily-crashed camera
service.

However, the error reporting for the those cases isn't really the
same, so switch to being explicit - check for the disabled camera
service system property, and if it's set, short-circuit calls.

Test: Camera CTS continues to pass, Watch device with no camera
      service also now passes camera CTS.
Bug: 62269118

Change-Id: I65a97f8c1b0f101999b2c04d4f1096b7f3aee858