Update preconditions to be asserts...

Which are now supported by Jack.
Retain a few preconditions checks that enforce contract.
Disable the one assert we had...because it failed in tests.

Change-Id: I2f6cb01e1af1534be4428ce4a3ee625a163dd163
diff --git a/Android.mk b/Android.mk
index e1650e1..6dfc3bb 100644
--- a/Android.mk
+++ b/Android.mk
@@ -30,6 +30,10 @@
   --extra-packages android.support.design \
   --extra-packages android.support.v7.recyclerview
 
+LOCAL_JACK_FLAGS := \
+  -D jack.assert.policy=enable \
+  -D jack.optimization.inner-class.accessors=true
+
 LOCAL_PACKAGE_NAME := DocumentsUI
 LOCAL_CERTIFICATE := platform