Fix a race condition during the rosalloc verification.

Since an exiting thread could revoke its thread-local runs without
holding the mutator lock, when rosalloc verification is running at a
pause, the thread could concurrently modify runs and cause a race
condition/verification check failure.

Now an exiting thread acquires the mutator lock when it revokes its
thread-local runs, which should avoid the race condition.

Bug: 18734151
Bug: 18577084
Change-Id: Ic8af558b96dff6fb6f5ace65351e2824a6ff4629