Deprecate ref... methods in MethodHandleInfo

These methods were mistakenly exposed as part of the public API in
Android 26. They do not appear to be used by any apps so this is a
prelude to removing them from the API completely.

Bug: 114175595
Test: make checkbuild
Change-Id: I17b7369a4fbb568bde5285d705d9194ac91e576d