SF: Introduce DisplaySnapshot

DisplaySnapshot stores the immutable state of a physical display,
captured on hotplug. The SurfaceFlinger class stores a map from
PhysicalDisplayId to PhysicalDisplay, a new class that augments
this state with the sp<DisplayToken>.

Later, DisplayDevice will be removed, and SF::mDisplays will map
wp<DisplayToken> to compositionengine::Display.

Bug: 241285876
Test: libsurfaceflinger_unittest
Test: dumpsys SurfaceFlinger --displays
Change-Id: I172df87c4a7a9cf64659ff9cfbd6df195ee4bc8d