camera2 api: Add CameraMetadata get/set support

* Add a Rational class
* Can get/set Key<T> where T is a primitive (or Rational)
* Can get/set Key<T> where T is a primitive array
* Can get/set Key<T> where T is an enum (synthetic constructor only)

Not implemented yet:
* When T is anything else, i.e. Rect, Size, etc

Bug: 9529161
Change-Id: I64438024a1e8327a38dd2672652626f0ffbb70e3