SF: Remove HWC plumbing for virtual display mirror

Replace the parameter, which is only relevant to SF-ARC, with a simpler
hook (virtual processDisplayAdded) that is closer to HWC-less overrides
for display creation.

Bug: 182939859
Test: Build
Change-Id: I9c8f84ec900d6538d9130c23a75ee79d1aefe13e