SF: Decouple EventThread from DisplayDevice

EventThread uses DisplayDevice::DisplayType constants, which will be
removed in a follow-up CL. This CL replaces them with local constants as
a stopgap until stable display IDs are propagated through the SF/WM
interface.

Bug: 74619554
Test: libsurfaceflinger_unittest
Change-Id: I68be363dc58c5e3aa17d05fba156d520a01fa775