Switch emulator to generate userdebug images by default

This is primarily to avoid using the unsupported JIT on Android-M

It will increase app install times significantly because the app will
be compiled at install time.  OTOH, It should improve application
performance dramatically.

Change-Id: Id1d637ec561db177fd52e4c111f4def2a1b45543
(cherry picked from commit 08787d747d71d7ff3f13d2275d937faefc6d7777)