Make stage split divider non-interactive while showing IME

Redirect IME inset control to wm shell during multi-window split. And
register IME position listener to make divider bar non-interactive while
IME showing.

Bug: 179262787
Test: manual check
Test: atest WMShellUnitTests
Change-Id: Ia83a29c823962221aedb1cc317928f6b4b7ddcae