Fix showing black background when resizing the divider bar.

Reports resizing state to WM core so that app client could apply bigger
visible bounds while resizing splits. Also moves DividerHandleView to
more generic package.

Fix: 172704238
Test: manul check divider behavior
Test: atest WMShellUnitTests
Change-Id: I1656c86de9a0b5dfe5e6da57ccc9d5f6e467843a