Slight polish for split screen gesture animation

* Divider bar dimensions manually calculated since
the leash that is provided has bounds larger than
the space that is visually shown between the two split
apps

Bug: 181704764
Test: Swipe up on large and normal screen,
w/ and w/o home rotation enabled

Change-Id: I1fde053151d47c6ce3e11f16f8ae4a153d273871