AAPT support for feature splits

This change allows the developer to add a base package for
which to build a feature split. The generated resource types
will begin after the base APK's defined types so as not
to collide or override resources.

Multiple features can be generated by first choosing an
arbitrary order for the features. Then for each feature,
the base APK and any preceding features are specified
with the --feature-of flags.

So with a base APK 'A' and features, 'B', and 'C',
'B' would be built with

aapt package [...] --feature-of A [...]

and 'C' would be built with

aapt package [...] --feature-of A --feature-of B [...]

Change-Id: I1be66e3f8df9a737b21c71f8a93685376c7e6780