Make presentation immersive

The main intention of this change is to ensure that all system bars
are hidden while the display is presenting. To achive this, this
change has two changes:

(i) Call WindowInsetsController#hide() in Presentation#show()
  We decided to apply this unanimously to all presentations.

(ii) Give presentation per-display focus when shown
  At least per-display focus is needed so that presentation windows
  can control system bar visibility. The easiest way to achive this
  would be to move the display to top and give global focus when
  a presentation gets shown, but unfortunately a presentation
  shouldn't steal global focus from its activity when it gets
  shown on another display, so with this change, only update
  per-display focus by triggering findFocusedWindow() when a
  presentation gets shown on a display.

Flag: com.android.window.flags.enable_presentation_for_connected_displays
Bug: 390481142
Test: atest PresentationTest

Change-Id: I90860f72363ef285f49674262d1c9a868c45f8e7