CallStyle: add flag for new action layout

Now that there's a bugfix flow for flags, we can use a real flag instead
of a makeshift boolean!

Bug: 268733030
Flag: ACONFIG com.android.systemui.new_call_style_action_layout DEVELOPMENT
Test: manual: disable/enable flag, post callstyle notif, observe actions
Change-Id: I6df281cc7d6850b9bc4cd156c699f970f3a661b5