Add reachabilityFence, remove ExemptionMechanism.finalize

ReachabilityFence is taken from Java 9.

ExemptionMechanism.finalize was never useful, and is also being removed
upstream. It is unsafe in the presence of dead reference elimination.

Bug: 63934467
Bug: 28342794
Test: Build and boot AOSP
Change-Id: I91dc4d33b91175248f44783e3c868a4f1b1d2d0d