Remove CameraAvailabilityListener TAG definition

Top-level const TAG should be private. Otherwise, it prevents other
files in the same package from defining their own top-level const TAG.
However, in this case the TAG is unused, so remove it entirely.

Test: m SystemUI
Bug: 238923086
Change-Id: Ie4a800526bab7d9a169b8e6c19616402daed5443