Add InstrumentFragment for non-PreferenceFragment types.

This allows app fragment use a less heavyweight fragment as super class
if they don't need PreferenceFragment. Using this class as base is
generally easier to set up robolectric tests too.

Bug: 33354536
Test: RunSettingsRoboTests
Change-Id: I91c4d242ea0333c76c8767c03c3f18dee6b6e104