Expand Pip rotate tests

1. Simplify transition

2. Split assertions for easier debugging

3. Add new assertions
    pip window above app window before/after rotation

4. Add documentation for test and assertions

Bug: 196213706
Test: atest WMShellFlickerTests:com.android.wm.shell.flicker.pip
Change-Id: I6f046678a5e1a4c7d42d0a3291b2c64294136c9c