Enforce rounded corners on App Widgets.

The feature is controlled by the ENABLE_ENFORCED_ROUNDED_CORNERS flag
(currently set to false).

If does not yet handle P/H flags to control its behavior.

Bug: 183097166
Test: Manual tests with top 1P App Widgets (See bug for results)
Change-Id: I56fca1b717f37ad518588115409f2144a71d4b98