fix an issue with vsync event delivery

vsync events were sometimes delivered to connected
client who didn't request them. this happened if
another client requested the delivery and that client
was first in the client list.

also fix the vsync test which didn't request any events as
well as DisplayEventReveiver documentation which was misleading
about the necessity to request vsync events.

Change-Id: Ie990fda3f337f8f0042745c4b2cde67936c45686