Add Start/Stop/Reset to CapturedStdFd.

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