Create gradle project when exporting sketch

[codeanticode]

The Export Android Project creates an ant project, we should switch to gradle which is the default adopted by Android Studio.

[jwill]

+1. I think maybe it should also bundle a gradle wrapper so that the resulting package can be run even if the person doesn't have Android Studio installed.

[codeanticode]

Implemented with cb99af6, 87f42d0, 7941f07, 9425bff, 6308d78, 730de21, b7b9b3c, 855f6b9, and e992ff0.

Outputting ant project is still possible by setting android.export.build_system to ant in Processing's preferences file. Gradle is the default now.