Camera: Updates in preparation for HIDL

- For all camera2 paths, and anything shared between the legacy API and
  camera2, switch to using String for camera IDs.
- Remove assumption that camera device IDs must be a dense set of small
  integers, and rewrite the camera ID query methods
- Change operation of ICameraService.addListener: it now returns the list of
  available devices directly, instead of invoking callbacks. This is
  needed to ensure an up-to-date list of cameras is immediately available
  on connection to the camera service.
- Add new CameraStatus object to use as the addListener return value
- Update tests to work with new interface

Test: cts-tradefed run cts -m Camera --skip-connectivity-check -d -o --abi armeabi-v7a --disable-reboot
Bug: 32991422
Change-Id: I6ff4a5dd014c6aefe0750850eeece6f3267a3e5b