[Flicker tests for bubbles]

Add new test as below for bubble notification to detect flicker
1. Dismiss bubble notification
2. Multiple bubble notifications switching

Test: 1. atest WMShellFlickerTests:DismissBubbleScreen
      2. atest WMShellFlickerTests:MultiBubblesScreen
Bug: 167521723
Change-Id: I2bbd458424e2121d6ab70a3dee170f77bf6a7066