Don't move PiP towards a corner after user manually moved it.

With keep clear areas flag on, PiP will gravitate towards one of the
bottom corners of the screen if possible. However, once a user moves
PiP manually, it will only continue to avoid occluding keep clear
areas, but no longer move downwards. This also happens if user resized
the window.

This can be disabled by turning the newly introduced sysprop flag.

Bug: 258529269
Test: manually

Change-Id: Ieaa292c54762d5cd005bcdf7eab48d6082e5c3a9