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
use gui/DisplayEvent to receive the events. Events are
dispatched through a unix pipe, so the API is compatible
with utils/Looper. see gui/DisplayEvent.h for more info.
Bug: 1475048
Change-Id: Ia720f64d1b950328b47b22c6a86042e481d35f09