Fix unwanted PIP window dismissal when moving to the bottom

Display#getSize() returns screen size excluding certain system decor
elements. We should get a more accurate size using Display#getRealSize()
minus bottom inset instead for calculating whether the PIP window
should be dismissed.

Fixes: 80367557
Test: atest CtsActivityManagerDeviceTestCases:ActivityManagerPinnedStackTests and manual test
Change-Id: I20cd8ccfa53bd7cc1cb7176280a01dbcb64b5745