Make taskview control into an interface [4/4]

Now that relevant control-level functionality has been
pulled out of taskview, we can turn TaskViewTransitions
into an interface (TaskViewController) and then have
different implementations of it.

Longer-term, it probably makes sense to also remove
the "forwarding" calls from TaskView to the controller
and just have the control-level manipulate the taskviews
as necessary. Once that happens we could probably remove
this interface altogether since it's only needed, now,
to support the inside-out calls.

The changes here are basically just renaming

Bug: 384976265
Test: refactor only, existing tests
Flag: EXEMPT mechanical refactor
Change-Id: Ie6de2228f873032f1cacdee125666c8d0e0cc33e