sdk: livedisplay: add a method to determine whether night mode is enabled

LiveDisplayManager.getMode() returns MODE_AUTO when night mode is enabled
automatically, so there's no way to determine if night is on when livedisplay
changes automatically

Change-Id: Ia47fc127232c2bb3b6634b55712060a919f8c9c1
Signed-off-by: Joey <joey@lineageos.org>