Fix pip expand tests (via button, intent and double click)

1. Rename tests
  Expand to app via intent
  Expand to app via button
  Expand to max size via double click

2. Improve existing assertions

3. Create new assertions, e.g.:
    pip layer expands
    pip layer in bounds

4. Add documentation for test and assertions

Bug: 171049504
Bug: 174238218

Test: atest WMShellFlickerTests:com.android.wm.shell.flicker.pip
Change-Id: I9bfad1ef97ddc31be9d897114e23d4645b4a63d5