DispSync: remove delay when enabling vsync events

This change fixes a bug that caused an extra frame of latency when enabling
vsync event callbacks in DispSync.  The bug was related to the logic that
prevents the two events from firing with very little time between them due to
updates to the vsync model.

Bug: 11479720
Change-Id: Ie7eaff9e92ffb7b7b6cb4d3d4402c96cbd29af7e
Merge "DispSync: remove delay when enabling vsync events"
Merge remote-tracking branch 'aosp/master' into kk44