Layoutlib: Load fragments

This change does the following:
- Make the bridge context extend Activity instead of Context
so that it can act as a view factory. This is needed because
the Activity is used as factory for Fragment objects.

- Override the default Fragment.instantiate(...) method
through a delegate. This is done to load the Fragment
classes located in the project (similar to custom views).

Change-Id: If62e7c9ff2b7585677077ad825aa1c3591d1b5e0