[Android] Add a bunch of gradle/AS related stuff to gitignore.

There are a LOT of files in the repository that probably shouldn't be. it seems like every time I fire up Android Studio, some metadata file indicating which tab I clicked on last Thursday is being edited, and I have to manually remove them from my commits.

What I've added to gitignore should suffice for most Android Studio projects. I know that just adding it to gitignore won't take it out of the repository, but I think someone who's better at Git than me should handle that.