Implement stretch-free resizing for PIP (input side).

This is a feature of two components:
1) Input code that takes user input around the PIP window border and
starts resizing
2) The actual resizing of the PIP task/stack that currently lives in WM,
but will live in SystemUI once migration is done to using Task Organizer

The CL only takes care of #1, the input side living in SystemUI.

Bug: 111426176
Test: Manually drag the corner of PIP window

Change-Id: If4b5e6ea5c482f4fa6eb571b441cf9e6966fbf4b