Move java.lang.ref.Reference.clear() to native

Required for adding exclusion for Reference.clear and reference
processing.

See accompanying change:
https://android-review.googlesource.com/#/c/313664/

Bug: 33389022

Test: test-art-host

Change-Id: Iee83b76d84453e929172f1a83f284aa4910e126c
2 files changed