Animate dialog stack and use in UserSwitcher

Allows to dismiss a dialog stack (dialogs launched from dialog content)
by dismissing the top one to the original touchSurface. After this, all
the intermediate dialogs are dismissed.

Use this in the user switcher dialog.

When adding users, the dialog that opens to set the name and picture is
not part of this stack (it's a separate activity), so the dialog stack
is collapsed before launching the activity.

Test: atest SystemUITests
Bug: 185360864
Change-Id: Ife8e0f9189783b69034ce660d1075c3f76fbbf06