SF: Register resync callback per event connection

This CL ties the resync callback to an EventThreadConnection instead
of an EventThread. This is a step towards having IDisplayEventConnection
subscribe to a given display rather than the primary display implicitly.
Each display will then have SurfaceFlinger::VsyncState that resyncs
independently at potentially different rates. Callbacks have weak
references to the per-display VsyncState owned by SurfaceFlinger.

Bug: 74619554
Test: Boot and turn display on/off repeatedly
Change-Id: Ic7cc64e2004fa07a5d54431fc330995048a4ed20