Prevent widget entries with duplicate widget_ids.

This changelist prevents the insertion of a widget entry where the
widget_id already is associated with an existing entry. This is
accomplished through two mechanisms. First, any insertion will cause a
deletion of any existing widget with the same widget id. Secondly, an
index has been added to enforce uniqueness on the widget_id column.

A migration has been introduced as well to analyze the existing items
within the database and clean up and corruption associated with
duplicate widget_ids.

Fixes: 433082016
Test: atest CommunalWidgetDaoTest#addWidgetWithSameWidgetIdMultipleTimes
Flag: EXEMPT bugfix
Change-Id: Ic83cb177c458dcb3767891eaea66e26b2cb9bf53