(Android) Add support for loading modules from multiple packages.

This is a solution to allow libretro cores to be added to an existing RetroArch install.  The android:sharedUserId attribute is set in phoenix AndroidManifest.xml, and ModuleActivity is patched to search all packages sharing the user id for modules.

The directory android/module-package has a framework (and a README to use it) for building and installing module packages.

This would primarily be useful to distribute cores which are either deemed too large, not mainstream enough, or not stable enough to be included with the main RetroArch package.  It might not be too useful for distributing third party modules in a release environment as all sub-packages will need to be signed with the same key as the main RetroArch package.

It can also be used to avoid copying all modules to the device when testing changes to native-activity.