re-organize recents transition code for future changes

Some code has to move around both for clarity and so that
future CLs can be more focused on logic changes.

The movements here are:

Create static functions for non-app and wallpaper tests used
to filter TransitionInfo changes to wrap with RemoteAnimationTargets.
This lets other code use the same checks.

Make a dedicate "LeafTaskFilter" that can also be used by
other code to select only leaf-tasks from a TransitionInfo. It
also attaches a semantic name to it rather than being an anonymous
lambda.

Moved recents setup logic into the wrapper, we were passing so
many parameters in and basically 100% of the logic is only used
by the wrapper anyways (and isn't relevant for general remotes).

Also made startNewTransition two-way because one-ways go into a
different queue so we can't guarantee ordering between startNew
and normal start.

Also renamed some variables to better communicate what they mean.

Bug: 269691848
Bug: 269691226
Test: no logic changes... existing tests pass
Change-Id: I67c153ed452a818f326c98c9a507251bd0c9eafa