[31/N] Desks: Consolidate desk activation utils

This change unifies some duplicated activation logic that existed in
both prepareMoveTaskToDeskAndActive() and addDeskActivationChanges().

prepareMoveTaskToDeskAndActive() was essentially doing the same work
that addDeskActivationChanges() did, plus calling
addMoveToDeskTaskChanges(). With this change, the prepare() function
now internally uses addDeskActivationChanges(), and is renamed to
addDeskActivationWithMovingTaskChanges() to follow the existing
addXChanges() pattern.

It also removes bringDesktopAppsToFrontBeforeShowingNewTask(), since
it's just an overloaded version of bringDesktopAppsToFront that
shouldn't be needed in kotlin.

This is a pure refactor, so no behavior changes are expected.

Flag: com.android.window.flags.enable_multiple_desktops_backend
Bug: 362720497
Test: atest WMShellUnitTests
Change-Id: Ife6b80c23f03b0f71a4b2f37c5f231df068e1035