Split PipShelfHeightTest to check both movement up and down

1. Split test into shelf moves up and down

2. Simplify existing assertions (using region subject)

3. Add new assertions
    pip layers moves up
    pip layer in visible bounds

4. Add documentation for test and assertions

Bug: 196213706
Test: atest WMShellFlickerTests:com.android.wm.shell.flicker.pip
Change-Id: Ia9a0a1d272dbe968098934292b214f6b6d31ed83