verifier: Bypass monitor-{enter,exit} depth checks

Per Google Bug 61916[1], the monitor-enter and monitor-exit checks in
dex2oat are excessively strict and will reject some valid APKs, such as
WhatsApp[2].  This change attempts to relax those requirements.

[1] https://code.google.com/p/android/issues/detail?id=61916
[2] http://forum.xda-developers.com/showthread.php?t=2525044

Amend from OmniROM: Add a proper ifdef variable that can be
toggled in Android.mk (-DWORKAROUND_BUG_61916)

Change-Id: I19ed1076b124b56ee158ede569fa26ac33910e1c
Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
Fix NewLocalRef, NewGlobalRef to handle cleared weak globals.

We were not checking for null after decoding the reference, this
meant that we incorrectly created null weak global references instead
of returning null.

Issue: 63929
Change-Id: I9159682e6edad8f415ef8144fc13b9aedd2cceb4