[CD Taskbar] Fix Window Context & Enable External Context adding

Use variable constrained within TaskbarManager for primary displays and add external window contexts as new displays are added. Ensures the lifecycle of window context map isn't tied to the required instances of primaryWindowContext.

Test: m
Bug: 395702003
Bug: 393984037
Bug: 391653300
Flag: com.android.window.flags.enable_taskbar_connected_displays
Change-Id: Idc461b8e6e249060feffac703dff1bf7d5974512