Acquire the mutator lock before comparing classes/strings.

Scratch my initial thought we woudldn't need it because the
handlescope is visited during the pause: as the compiler thread
is in state native, the GC can concurrently update the handlescope,
leading to false negatives when doing class/string equality.

bug:34240874
test: test-art-host gcstress
Change-Id: Icda0722fb49300a7de57e1c5d1efaa9e8dbda83f