camera2-portability: Make factory offer choice of backing API

This allows apps using the library to decide whether to jump to the new API.
Users of the library may choose to use the Camera class, the camera2 API, or
have the choice made automatically based on the device's API level. The choice
may also be globally overridden by means of a system property.

Bug: 16875535
Change-Id: I92e4be96fbd04057be61ceb8a0c58935770022f6