Extensions to check JNI.

Ensure critical lock isn't held when returning from a down-call.
Log a warning if the critical lock is held for a significant period of
time.
Refactor JNIEnvExt to be a class rather than a struct.

Test: mma test-art-host

Change-Id: I4d149cb04d3a7308a22b92b196e51e2f1ae17ede