Save usercreated bubbles and re-bubble next time; add experiment debug logs

* Maintain a list of user-created bubbles in BubbleController
* Experiment code will allow any notif with a matching key in that list
  to be created as a bubble, does not adjust any channel level setting,
  does not maintain across reboots
* Add some debug logs (turned on by default, only triggers if experiment
  via set prop is running)

This is more of a stop-gap for testing until we have proper way of saving
that this person/notification from app should be bubbled or not.

Test: manual
Bug: 143173197
Change-Id: Id247e361740cdf95ae511287ad58c547825f1e47