SF: Store and manage snapshots for virtual displays

Create/destroy VirtualDisplaySnapshot that tracks the virtual display's
lifetime.

For now, VirtualDisplaySnapshots will track a virtual display's
DisplayId and whether or not it's backed by Hal/Gpu. This will be
helpful when moving the cast system of DisplayIds from within
DisplayId.h to SurfaceFlinger.

Bug: 374163881
Flag: EXEMPT refactor
Test: libsurfaceflinger_unittest
Change-Id: Id730e65b24ecd10925282bd72585ac7c5635a4fe