Add SystemModalsTransitionHandler to deal with system modal transitions

Before this CL, the launch and close transitions for System Modals
(e.g. add-shortcut-to-homescreen dialogs and bluetooth pairing dialogs)
were animated using DefaultTransitionHandler.
The animation in DefaultTransitionHandler doesn't work well in certain
cases for Desktop Mode, where not just the modal dialog, but also the
outline of its backing Activity, is shown during animation.

With this CL we apply a simple alpha-animation for launch and close
transitions of System Modals when in Desktop Mode.

Test: SystemModalsTransitionHandlerTest
Bug: 335638193
Flag: com.android.window.flags.enable_desktop_system_dialogs_transitions

Change-Id: Icad12a4dcf328ce2d65792e514f7493d5a535335