SF: Remove fallback for outdated active display

Initialize mActiveDisplayId during boot. getDefaultDisplay's fallback is
then no longer needed, since mActiveDisplayId is an internal display, so
cannot be removed and become outdated.

Bug: 255635821
Flag: EXEMPT refactor
Test: Boot
Change-Id: Ifa1a1f679957e88788b1819ba117fda3f407430b