Fix enter pip via UI tests

1. Improve existing assertions

2. Create new assertions, e.g.:
    pip layer reduces
    pip layer in bounds

3. Add documentation for test and assertions

Bug: 174238257

Test: atest WMShellFlickerTests:com.android.wm.shell.flicker.pip
Change-Id: Ib08aa3e2f4dfe5112cca181ffddd301469354f5e