Add support for building split APKs

Build multiple APKs, each containing a disjoint subset
of configurations. These can then be loaded into the device
AssetManager and should operate as if they were never split.

Use the idea of building multiple sets of files, where each
set represents an APK. An ApkBuilder can place files
in a set based on its configuration, but you can actually
add directly to a set, in the case of the resources.arsc and
generated AndroidManifest.xml for splits.

Change-Id: Ic65d3f0ac1bbd290185695b9971d425c85ab1de3