[6/n] Tile resizing handle core business logic for dragging and animation

This CL introduces onDividerHandleMoved which initiates and updates the
veil for both tasks simultaneously, then at the end of drag, initiates
a WindowContainerTransaction request to resize both apps to the new bounds.

Flag: com.android.window.flags.enable_tile_resizing
Test: tests are WIP
Bug: 369341677
Change-Id: I906f87ca260eb598211e31a708ce3f505cdf8148