Move the fd() function to be private since it should not have been
exposed in the first place.
Fix the way logging_test uses CapturedXXX.
Adding this because the new isolated testing doesn't print errors to
stderr so the ASSERT_ EXPECT_ messages can get swallowed. Also, it's easier
to reuse a CapturedXXX object in a test with these functions.
Test: New unit tests pass.
Change-Id: I38b113fc184146ce434802f80a9b7997fa83e78a
eea85c9aab