PiP: Take inset into consideration when calculating stash offset.

When stashing, currently it assumes the entire display is usable - not
taking into insets into consideration that can be caused by navigation
bar, etc.

Bug: 186698828
Test: Manual. Stashing PIP while w/ insets now stashes correctly.
Test: atest PipSnapAlgorithmTest
Change-Id: Ia6cbb8f21b666a4ba4ee10b84e3087b6a23cd982