Make Barrier robust against spurious wakeups

Fix Barrier implementation so that Wait cannot return prematurely
due to a spurious condition variable wakeup or interrupted futex call.

Document the resulting semantics of barrier.h, which are a bit surprising,
but appear to be exactly what current clients need.

Fix the test so that it actually passes with the fixed barrier.h,
and no longer tests for properties that can't be correctly satisfied.

Improve comment for InitTimeSpec, which we almost used.

Bug:18509123

Change-Id: I0b25f33bcd22322ba04e3951cd484843788c2bf5