Introduce DesktopModeHelper#isDesktopModeSupportedOnInternalDisplay()

When persist.wm.debug.desktop_mode_enforce_device_restrictions is set to
false, the internal display can host desktop mode, and the connected
displays should be treated as extended mode rather than projected mode.

DisplayManager should use this new API to differentiate extended and
projected mode, replacing
DesktopModeHelper#canInternalDisplayHostDesktops().

DisplayManagerService will use this new API to decide if the default
display should be excluded from the topology by default.

Bug: 422292780
Test: passed the modified unit tests
Test: adb shell setprop
persist.wm.debug.desktop_mode_enforce_device_restrictions false and
checked the display topology
Flag: EXEMPT bug fix

Change-Id: I1f5325b4b3bffae4fc74b9bdaf6cedce8aaf853d