Fix ConditionVariable.block to use elapsedRealtime.

System.currentTimeMillis() is NEVER okay to use to calculate a time delta.
The original spirit of this probably should have used SystemClock.uptimeMillis(),
but use SystemClock.elapsedRealtime because it's the same as
System.currentTimeMillis() in all the cases where System.currentTimeMillis() is
correct.

Test: Treehugger
Change-Id: Ib6e090425af83e7e822fb12814d87e8a37e2d2ac