[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.
Don't reference some "old .gitignore". This comment doesn't make any sense if someone just looks at the file without any relation to git history or something like that.