Add ReadViewCameraViewController/Mediator.

- Launch config_rearViewCameraActivity in the overlay Window when
  gear is reversed.
- Add/Remove ActivityView dynamically to save the system resource
  while it is hidden.

Bug: 157481389
Test: atest com.android.systemui.car.rvc
Test: Check if RearViewCamera is launched when the gear is reversed and if it will be closed when the gear is changed to the other one.
Change-Id: I768fe9090eed9cb82c4108ab8f6335802d36fa64