Add grouping

If a set of AutofilledTextViews is grouped then the AutofitHelpers do not set the text sizes directly, but store the text sizes in their parameter fields mSizeToFitTextSize (in fact it always store) and notify the AutofilGroup, which checks all group members and applies the smallest text size to all grouped AutofitHelpers.

If the AutofilledTextViews are not grouped they can be used exactly as before.