[CD Taskbar] Make External Taskbar 3-Button & Persistent

The gesture handle still persists in conjunction with the three buttons. In addition to being forced into three button mode, the taskbar on external displays needs to never enter transient mode. More info and before/after images on bug.

Flag: com.android.window.flags.enable_taskbar_connected_displays
Bug: 399718805
Test: Manual
Change-Id: Iff5c297c8ac4823fa24e7a8e4becd0447224cac0