Add background executor to Bubble class

Adds background executor to Bubble class to prepare moving
BubbleViewInfoTask from using deprecated AsyncTask for bubble view
inflation. Use the shell background executor instead.
We are experiencing some bugs with bubbles showing up and the cause is a
race condition in view inflation.
Using an executor instead of the AsyncTask is the first step to fix
this.
Executor will be used for inflation in a follow-up cl.

Bug: 353894869
Test: atest WMShellUnitTests
Test: atest BubblesTest
Test: atest BubbleEducationControllerTest
Test: atest BubblePositionerTest
Test: atest BubbleStackViewTest
Flag: EXEMPT refactor

Change-Id: Ibb936cd649701ed5f16d5d8a3e340c8aeb11b52c