SF: Remove fixed-size array for VSYNC events

EventThread maintains two DisplayEventReceiver::Event instances to store
the latest VSYNC state for the internal and external displays, regardless
of their connection status. This CL removes the (unused) latter instance,
replaces the former with VSyncState, and generalizes event processing by
queuing up VSYNC events along with hotplug events.

The VSyncState lifetime will be tied to hotplug events in a future CL.

Bug: 74619554
Test: libsurfaceflinger_unittest
Test: dumpsys SurfaceFlinger --vsync
Change-Id: I5fbc1d08259145387dab73596a0cfe4624c35676