Update CI

This hooks checkstyle into the gradle script itself so you don't have to deal with extra scripts/makefiles.

Also sets up the CI to actually build the projects as well.

The build failed with a bunch of checkstyle exceptions. Some are from pulling in BitmapCache into the project which I can simply exclude if you want, but a fair few are from the core project. I didn't want to remove and checks or do a heavy change in the code before confirming first.

Do you also have a code style config for IntelliJ/Android Studio? Might help with making the project more consistent.