Break out CameraClient from CameraService.

To allow for different low-level implementations of the current camera
API, the Client class inside CameraService has to be inheritable. This
change breaks it up into the common section accessed by the camera
service, and the full implementation for camera device HAL version 1.

The full implementation is moved out to CameraClient.h/.cpp.

There are no functionality changes, just code reorganization.

Bug: 6243944
Change-Id: I8a79b31a3ae3617198dc6bb5576a9f2e2d9c95de