Delay PiP movement in response to keep clear areas changed.

This prevents erratic movements caused by animations that update layout each
frame, effectively reporting new keep clear area bounds every frame and
triggerring the movement multiple times. This will now wait for the keep
clear areas to stabilize before moving away.

Can be customized via system property.

Video before: http://recall/-/g8x7ZkgdfbqscttAA9wzl9/hwr0OrO4Pq8cvLw9bgwLff
Video after: http://recall/-/g8x7ZkgdfbqscttAA9wzl9/bGTKo0oqCKUwtvBdBYOB9w

Bug: 183746978
Test: manually, e.g. open PiP and slowly swipe up to go to overview, which
causes Hotseat to animate away triggering keep clear areas changed multiple
times, but the PiP moves only once

Change-Id: I2214e2250fb3cace8fee117285fd8ffbd27a3870