DispSync: Actually wait forever

When computeNextEventTimeLocked returns INT64_MAX (because there are
currently no listeners), wait until the condition variable is
signaled instead of waiting for a long timeout. Since the condition
variable timeout is specified using CLOCK_REALTIME behind the scenes,
we can receive false wakeups in the presence of large system clock
changes.

Bug: 28152577
Change-Id: I88dbab5d5d0776cb25dea76a4574db055b308fd1