libcameraservice: Refactor the CameraServiceProxyWrapper into a class.

This is necessary for writing a unit test for camera permissions that
checks behavior when the device policy manager has disabled the camera.
Using the true DevicePolicyManager service would require a test be run
while the device has a work profile - a much simpler method would be to
fake the results from CameraServiceProxyWrapper::isCameraDisabled with
an override.

The CameraServiceProxyWrapper is now managed by a shared_ptr that
originates in CameraService, unless this is overridden by an outside
source (cameraservice_test). As a consequence of this, some refactoring
has been done to CameraSessionStatsWrapper class to avoid circular
ownership between it and its parent (CameraServiceProxyWrapper).

Change-Id: Iad4ee87f5c5ab849eee65b72f46f77faf37f4d25
Test: CtsCameraTestCases