SF: Use consistent names for display IDs

There are two types of display IDs: hwc2_display_t for HWC IDs, and
int32_t for SF IDs. The latter are confusingly referred to as "hwcId".

This CL renames them to "hwcDisplayId" and "displayId", respectively.
Note that int32_t will be replaced by DisplayId in a follow-up CL.

Bug: 74619554
Test: Build
Change-Id: Ia9b621d385c2bf842a2093b578140dc7a2eaea69