Add missing startHomeActivity call during user-start..

When switching to a foreground user whose top task should be home. This
fixes a regression from Ib42d707d0f5c279042080c004044667a11c74f0c that
causes the SetUpWizard to not show for newly created users.

To include this activity start in the same user-switch transition, this
change also refactors UserController and WindowManagerService such that:
1) The early call to WMS#prepareUserStart (formerly #setCurrentUser)
   does not make any changes to window containers. Those will be made
   later when the transition is requested. Instead it runs other side
   effects that must happen early in the user-start flow (such as
   WindowManagerPolicy#setCurrentUserLw that must be sent before new
   user id broadcasts are sent).
2) All window container changes that were spread out before
   (RootWindowContainer#switchUser, DisplayContent density, resume top
   activities, startHomeActivity) are now wrapped be the user-switch
   transition started in WindowManagerService#startUserSwitchTransition.

Flag: com.android.window.flags.apply_desk_activation_on_user_switch
Fix: 439970292
Test: flash device, set up default user, go to Settings and change
display size, then create a new user and switch to it - verify the SUW
is started instead of skipping it and going directly to the launcher.
Cherrypick-From: https://googleplex-android-review.googlesource.com/q/commit:2b597ab1a6bfc28b802b39c2e97cdff9b8df5917
Cherrypick-From: https://googleplex-android-review.googlesource.com/q/commit:844ddc15f923319b6ec9e1c84dc3ed1778caf7fa
Merged-In: Id287d4ffb038c27c6e7dfbe472d64129a19b887c
Change-Id: Id287d4ffb038c27c6e7dfbe472d64129a19b887c