Allow building only split APKs

[lberrymage]

As per the default behavior of bundletool, standalone APKs are generated when generating an APK set for an app with minSdk < 21. This is not always desirable since the publishing app store may not support such old versions of Android and so will waste disk space since split APKs are sufficient for its needs. There should be an option to override this behavior and only build split APKs even for apps with a minSdk < 21.

The easiest (and most likely best) way to override this behavior is simply to override the app's minSdk from this plugin. This also has the advantage of solving the related (albeit less severe) issue of split APKs with compressed native libraries being generated when uncompressed native libraries are supported (the issue is less severe since it doesn't often produce nearly as much wasted disk space).

Upstream issue: google/bundletool#309

[lberrymage]

An alternative (and perhaps a better one) to modifying the minSdk is adding options to the plugin configuration such as stripCompressedDex and stripStandalones which enable an extra step to remove the corresponding files from the generated APK set.