Fix a dumb error allowing isThemeable=true for themes.

This issue caused problems attempting to access the resources from a
theme package by using Context#createPackageContext.  If the package
context was marked as themeable, the resulting AssetManager would
include both the requested theme APK as well as the global theme APK
which would conflict.

Change-Id: Ieece5fc4e5d4e7d50bbd16f9165afce4c09c4ea2