New trick to install bad dex file.

Previously we use LOCAL_JAVA_RESOURCE_FILES to override the normally
generated classes.dex with a bad dex file.
Now as we have moved Java resource processing before running jarjar, dx
would fail for the bad classes.dex inside the classes.jar.
Instead we override the target specific variable PRIVATE_DEX_FILE to
inject the bad dex file.

Bug: 18837479
Change-Id: I73f82c8a15e1d8935d9c2f290ba9456184a56ec7