EventThread: wake for two frames after vsync request by default

The common case seems to be that many frames are rendered in a row
before the system quiesces, but EventThread assumes that only a single
frame will be rendered after a call to requestNextVsync. This patch
moves requestNextVsync to wake for two frames by default, allowing it to
skip the EventThread wakeup when there's a pending wakeup for that
EventThread already.

Test: no EventThread wakeups from requestNextVsync in BouncyBall
bug: 169368457
Change-Id: I5cb880dad27062e158de640540a73cba59b223e5