Let System UI show on external displays (1/3)

Let each display have one status bar and one navigation bar. This is
so on each display, status bar and navigation bar can be laid out with
apps and produce proper insets.

Bug: 117474929
Test: atest com.android.server.wm
Test: Watch YouTube video in fullscreen mode, and see if status bar
      and navigation will be hidden as expected. Swipe on the edge
      of screen and see if status bar and navigation bar are both
      shown as expected.

Change-Id: I1550659b7cd1dd1676bf04483c5b68376ef42905