Fully qualify the namespace of the Fragment class we are using

If we just inherit from 'Fragment' or one of it's children and the file has a using statement of
'Android.App' and 'Android.Support.V4.App' the App version will win if targetting an Android
version that supports native fragments.
Update using statement
Merge pull request #822 from kjeremy/fix_support_version

Fully qualify the namespace of the Fragment class we are using