Consolidate split layout adjust animation for showing IME panel

Fix the getImeTarget always return null task target. Also make sure to
restore and prevent it adjust the split layout after IME is no longer
attached to any of split surface.

Fix: 224882069
Test: atest WMShellUnitTests
Test: toggle IME panel on the bottom split, verified it'll scrim top
      split and push up the split layout.
Change-Id: Icd0647db08a06d0401fc3c8c82929223a1bdcf7d