Use gradle to build processing-core

[codeanticode]

Recent versions of all Android dependencies (wear, cardboard) come in .aar format, which is not supported by ant.

Although it is possible to extract the libraries from the aar files and package them as jars, as explained here, it is a cumbersome process that increasingly harder to keep the dependencies in processing-core up to date.

The most convenient solution seems to move over to gradle in processing-core, while still using ant for the mode itself. This PR will definitely be useful.

[codeanticode]

Implemented by merging and tweaking PR from @jwill d7dd220