Adjust split layout with IME animation in split (3/N)

Listen to IME position changes in SplitLayout to make sure it updates
divider view status after re-inflated. Also add functions to get IME
position of split which is needed for integrating IME animation with
new split implementations.

Bug: 179262787
Test: atest WMShellUnitTests
Test: observed divider bar won't change to interactive after rotating
devices with IME shown.

Change-Id: Ic01c23e61ffb666cfc292e56dd6dcf4646be749b