Persists bubbles to disk (part 4)

1. Decouple Bubble from NotificationEntry
2. Convert ShortcutInfo into Bubbles
3. Load overflow bubbles into memory

Test: follow the following steps
1. add a few bubbles from test app
2. restart the device
3. add a bubble from test app
4. click the plus sign to expand overview bubbles
5. verify overflow bubbles is displayed properly

This reverts commit b28e0568.
Reason for revert: Rolling-forward with fixed unit test

Bug: 156697897
Bug: 149713060
Change-Id: I21e4b17ee53ee7b6698115e1b497253bb54b355f