Removed guava from runtime dependencies

I did 2 things:
- changed android.version and android-support-v4.version to the latest versions found in the central repo
- copied Preconditions.checkState to this project and removed guava from dependencies
It is still there as a transitive compile dependency, but if clients don't depend on it themselves, their apk size just dropped by 2.2 MB. This should be notable in testing, because in ADT proguard isn't run for test apks.