allows referencing of android support libraries and play services, which live within the user's SDK directory.
There is currently ANT-specific logic within plugman to handle them, and they don't work at all for gradle projects.
(note the use of local.properties)
Some example tags:
Currently (for ANT), these libraries were added as sub-projects using the path to them within the SDK directory. For gradle, we should be added them via:
Look for `src` in the form of `extras/android/support/v4` and map that to `com.android.support:support-v4`. Just use a hardcoded lookup table, as this is just for backwards compat.
Add support for `src` in the form of `com.android.support:support-v4`, and add these to the project.properties file, in the same way that we do for subprojects and gradleReference frameworks.