Unify Window resize (1/N)

This decouples the Split and Pinned stack resize flows. The
current goal is to prepare for a leashed resize implementation
for split-screen. For this to work, it needs to be clear
what actual logic belongs to the various docked resizing modes
vs what is for general stack resize or pinned resize. This
also enables PiP and Split to be worked-on in parallel.

As can be seen, general stack resize is actually not required
because freeform and fullscreen modes use task resize.

Bug: 119687367
Test: go/wm-smoke + related CTS
Change-Id: I9cca1910a18b87dda618235e89f2bdc582086f34