Fix deleteDatabase() for Android

Adds a unit test that ensures that we can delete a database, data is actually deleted, and then also that `deleteDatabase()` returns an error when the database doesn't exist.

I tested on iOS 7, Android 4.4, and Android 2.3, and the tests pass.

This is rebased on top of my other PRs because I didn't want to have to manually merge everything in later.  :)