Inflate & make bitmaps for bubble views in background

* Simple AsyncTask to load bubble content
* BubbleController finds out about a notification being posted, if it
  is a bubble, BubbleController creates the bubble & tells it to "load"
  once it's done, BubbleData does its thing as per usual
* Anywhere we need to "reload" the bubbles (e.g. theme change) should
  use the async task
* Updates tests to work with these changes

Test: atest SystemUITests
Bug: 144719337
Change-Id: If55f27a517bff0c1f467722966a7b3b7075e9403