Fix and re-enable CMS

Fix for CardTable::Scan double length scan which caused occasional assert failures.
Fix for the case where CMS request JNI code caused a GC which freed newest allocated object.
Fix error with is_gc_running_ never being set and deadlock due to CV wait without thread state change.
Fix error where we were Unlocking the heap before doing ResumeAll in CollectGarbageInternal which resulted in a rare race condition.
Fix error where GC could be in progress when the runtime terminated.

Change-Id: I8d243d4dca63170433a63429442788ecb09e55fe