Is there any reason to favor Python or Java over the other for developing on Android phones, other than the usual Python v. Java issues?
Java is "more native" on the Android platform; Python is coming after and striving to get parity but not quite there yet AFAIK.  Roughly the reverse situation wrt App Engine, where Python's been around for a year longer than Java and so is still more mature and complete (even though Java's catching up).
So, in any situation where you'd be at all undecided between Java and Python if the deployment was due to happen on some general purpose platform such as Linux, I think the maturity and completeness arguments could sway you towards Python for deployment on App Engine, and towards Java for deployment on Android.
Android: Java v. Python