SF: Use VsyncSchedule's ID for EventThread

The existing code will take the first display that is hotplugged and set
it as the PhysicalDisplayId associated with the EventThread's future
vsync events. There isn't anything wrong with this in the current
implementation because SurfaceFlinger is careful about removing/adding
displays when pacesetter is changed.

The EventThread is already associated with a specific VsyncSchedule. The
VsyncSchedule already maintains the PhysicalDisplayId. Use that instead
and make the code slightly less fragile.

Bug: 352324977
Flag: EXEMPT refactor
Test: presubmit
Change-Id: I33d7da120289e5cf1127d783eae3011280bf65bd