Camera: Make CameraMetadataNative compile-time initializable

Ensure that all static initializer code can be executed in the
compiler. Move native field caching into the native method
registration function.

Saves up to 10ms on N6P zygote initialization.

Bug: 34956610
Test: m
Test: Device boots (N6P)
Test: Can take pictures and videos (N6P)
Change-Id: I481dfd3aa8dc3cc5a5c31eedad31390d8afda897