Don't allow suspension from FindLocksAtDexPc

Transitioning to suspended from runnable sometimes runs dump
checkpoints in ThreadStress which can cause deadlocks. This happens
since FindLocksAtDexPC runs the verifier which calls
AllowThreadSuspension. This results in a blocked thread which holds
the monitor lock, and if another thread tries to do a monitor enter,
it deadlocks while holding the mutator lock (assuming the GC is
suspending all).

The fix for avoiding this deadlock is not calling
AllowThreadSuspension from FindLocksAtDexPc.

Bug: 18576985
Change-Id: I7e5faaf3bbbd5b5f680de95d53c33b5106705b0c