Work around the timing resolution of Handler#postDelayed()

This is a follow up CL to my previous CL [1], which added unit tests
for CompletableFutureUtil.

It seems that the time resolution of Handler#postDelayed() is indeed
milliseconds, and one may observe that the scheduled task can be
triggered a bit earlier within the resolution.

This CL introduces a utility method so that all the tests in
CompletableFutureUtilTest that rely on Handler#postDelayed() can take
that behavior into account.

There should be no semantic changes in the test expectations.

 [1]: I215bbc870f952effa262fa431064b36ace28e8f4
      ba8cdf5d

Fix: 198735181
Test: atest -c FrameworksCoreTests:CompletableFutureUtilTest
Change-Id: I70f6c80398d9f97d17764a2aebbc86d9669a0272