Trebuchet: protect update path for widgetpreviews db

After the update path is called, calling getWritableDatabase
always results in a SQLiteReadOnlyDatabaseException. Looks to
be a Lollipop bug, protecting the code path till we have a
better solution.

Change-Id: Ic0487e2d588a341d42e046fb97e8f041622d6450
(cherry picked from commit 368c91bd)