Don't allow removed DividerView to resize stack

If a divider view is removed or in the process of been removed, then it
should no longer have any influence on the size of stacks.

Fixes: 62273599
Test: go/wm-smoke
Test: enter split-screen and make sure recents is visible, lock and
unlock the device and verify things look fine.

Change-Id: I79aca82be8a7013328b78e82f508c4ed9fa48a4a