Camera: Advertise numbered string ID for external cameras

With current implementation cameraId is part of something like:

  device@3.5/external//dev/video0

This doesn't work well with Camera2 API based apps.

Adding CameraIdOffset tag to Provider as a base offset.

Test: presubmit builds
Bug: 151350336

Merged-In: I6309d16be565616f048fb24a70e9be5b2f5ed480
Change-Id: I6309d16be565616f048fb24a70e9be5b2f5ed480
(cherry picked from commit 4f6022ae)