Add infrastructure for registering a native frame metrics observer

This will allow NativeInputEventReceiver to subscribe to the frame
metrics notifications directly from the native code, without having to
involve java.

Java would still be needed to register the observer, though.

Bug: 376713684
Test: compile only, this infra is used in another CL
Flag: EXEMPT refactor
Change-Id: I7be8f0b2e94595f9a64ce69a22fe5910e0cd7711