3.4 Update ActivityStarterInternal implementations.

They now branch to use the new ActivityTransitionAnimator APIs when
the flag is enabled.

Trying to make each of the CLs as small as possible to keep them
digestible and low risk. For the refactor plan see
go/animlib-shell-refactor-plan.

Bug: 397180418
Flag: com.android.systemui.animation_library_shell_migration
Test: atest ActivityStarterInternalImplTest LegacyActivityStarterInternalImplTest + manual
Change-Id: Ic04a5e67f9e8774ce8898cd3dc623efad9c84f64