Move some display logic into hierarchy [1/2]

Starting to decouple WM-side computation logic from specific
hierarchy levels in WM hierarchy. This means trying to make
helper functions in pinned/docked controllers and in TaskStack
more "static" by using parameters instead of mDisplayContent
references. The short-term purpose of this is to make it
easier to use this logic from the ATM hierarchy. Longer-term
the purpose is to make it possible to use the controller
logic at any level of the hierarchy.

Bug: 113900640
Test: go/wm-smoke + relevant am/wm servicestests
Change-Id: I7054a0d4d75862bb152bae9bc179a418a2dadffc