SF: Remove layer stack lookup for screencap

GPU virtual displays now have IDs, so the (ambiguous) fallback is no
longer necessary.

Remove PhysicalDisplayId constructor from uint64_t to enforce error
checking at compile time when upcasting or deserializing.

Bug: 162612135
Bug: 182939859
Test: screencap -d <id>
Test: DisplayId_test
Change-Id: I68fa0835cbc915975c7fa40c7d544d9491db9fa2