Let window manager know about active camera users, part 1

- Each time a new app starts or stops using cameras, inform the
  window manager from the camera service proxy
- If an app uses multiple cameras, only notify on the start of the
  first camera use, and at the end of the last camera use.
- Actual implementation of window manager internals is next;
  temporary logging is in place for testing.

Test: On Pixel 3, verify that temporary logging appears as expected
  when apps open the camera and close it; verify multi-camera case by
  using TestingCamera2.1 to open/close camera IDs 2 and 3, which can
  be operated concurrently on Pixel 3.
Bug: 130744843

Change-Id: Ie808507fa7f35591cb19fee032413d6fb7700c9d