Don't add or remove verifiers if aborting

Prevents deadlock if marking verifier roots fails when we attempt to
dump the stack traces. The deadlock occurs from FindLocksAtDexPC
since this allocates a new verifier then adds / removes it from the
method_verifiers_ array.

Bug: 18651054
Change-Id: Ia9b9470ce5e4ac20bfbb39bef0283974cf487765
Merge "Don't add or remove verifiers if aborting"