Remove docked task drag resizing mode

Clean up the no longer used "docked" resize mode previously used by
legacy split-screen. That leaves only one drag resize mode (freeform),
so the concept of "modes" is also removed in favor of a boolean
dragResizing flag.

Bug: 249808500
Test: atest WmTests:WindowStateTests
Change-Id: I41eea449ed2682d7a10d468bbd04eb9ebd84d4ff