Fully qualified name for R class, allowing Activities in subpackages

As mentioned in #1, there's a bug when an Activity isn't in the same package as the generated R class. This pull request fixes that. 

**Note:** the bug still happens if the R class doesn't belong to a parent package of the activity, like `org.foo.R` and `com.bar.activities.MyActivity`. In that case, the generated java file would have the same error as the version without the patch.