Camera service: Add skeleton support for camera2 devices.

- Add Camera2Client, which maps the current android.hardware.Camera
  API to the new camera2 semantics
- Add Camera2Device, a C++ wrapper for the HAL camera 2 device

No functionality besides create/destroy; all client methods return
error or null.

Bug: 6243944
Change-Id: I6bd5136fa1efd8279b8cd2d9847d9b0bb2862294