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

Bug: 149713060
Change-Id: I6905d93fe409f4401f7d5d2d08597c36c028b5de