Widgets recommendation backend

Add a widgets recommendation mechanism based on AiAI app predication
ranking with the following changes:

1. Only one widget is picked from one app.
2. Widgets that are already added to the workspace are excluded from
   the recommendation.

Test: run PredicationUpdateTaskTest

Bug: 179797520
Change-Id: Ia697bc6df0bae75969e68b7b3de32d57901f7461