Reset the stream state at the end of each line of logging.

The Android logging always behaved like this (because there's a new
underlying std::stringstream each time), but on Linux we were mutating
std::cerr.

Change-Id: I991cb769f783f82aea53ec023e3aea330a1a304c
diff --git a/src/logging.h b/src/logging.h
index 5169e2a..bf6d179 100644
--- a/src/logging.h
+++ b/src/logging.h
@@ -124,6 +124,7 @@
 
  private:
   std::stringstream buffer_;
+  std::ios_base::fmtflags stream_state_;
   const char* file_;
   int line_;
   LogSeverity severity_;