Correct API for new sizes in App Widgets.

This follow recommendations from the API council review.

Based on those recommendations: I updated the API and updated the
comments to make the function behavior clearer.

Bug: 181611658
Test: atest android.widget.cts.RemoteViewsSizeMapTest
Test: Local widget to check rendering
Change-Id: Ie9fcedbc7b18b83f6d1220f99240f264e53e3649