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
diff --git a/include/android-base/test_utils.h b/include/android-base/test_utils.h
index 9e2ea97..2abe68e 100644
--- a/include/android-base/test_utils.h
+++ b/include/android-base/test_utils.h
@@ -62,16 +62,18 @@
   CapturedStdFd(int std_fd);
   ~CapturedStdFd();
 
-  int fd() const;
   std::string str();
 
- private:
-  void Init();
+  void Start();
+  void Stop();
   void Reset();
 
+ private:
+  int fd() const;
+
   TemporaryFile temp_file_;
   int std_fd_;
-  int old_fd_;
+  int old_fd_ = -1;
 
   DISALLOW_COPY_AND_ASSIGN(CapturedStdFd);
 };