Add a display flag for maintaining its own focus and touch mode.

This basically duplicates the behavior of config_perDisplayFocusEnabled
but via a display flag instead of globally for all displays. The display
must be trusted for the new flag to be set.

This is particularly needed for virtual devices and displays with
navigation-type inputs, for which touching the default display will
cause everything to enter touch mode and therefore the currently focused
view would lose focus.

Bug: 233911853
Bug: 256810557
Test: atest DisplayManagerServiceTest
Test: atest WindowManagerServiceTests
Change-Id: Ie1c3b8a42f512530bf0ce3fabc56f0da236a311c