Fix AAPT-compatible output format for locales

The previous code used a format that is not supported by AAPT,
although it claimed it was. The new code generates locale codes in
the format AAPT can consume, using xx and xx-rYY for simple locales,
and the b+ format for everything else.

Bug: 25599046
Change-Id: I34d432f23b84913d313d6562c110d7fad87c457a