SF: Define DisplayId as struct type

This prevents accidental mix-up with HWC display IDs, which have the
same backing type, as well as other implicitly convertible integers,
e.g. EDID manufacturer IDs. This CL also fixes misuses detected by
type checking:

1) Recycling of virtual display IDs.
2) Skipping composition for non-HWC virtual displays.
3) Unit tests for fallback/virtual display IDs.

Bug: 74619554
Bug: 119412688
Test: libsurfaceflinger_unittest
Test: vrflinger_test on walleye_xr
Change-Id: I0be41cc93c82860e859f1adf427430436c926595