Fixed WV metrics YAML formating.

[ Merge of http://go/wvgerrit/171270 ]

The metrics dump tool for the Widevine DRM factory was producing
malformed YAML output.  This is compounded by the formatting using
fixed indentation for its output.

Bug: 239462891
Test: adb shell dumpsys android.hardware.drm.IDrmFactory/widevine -m
Test: atest GtsMediaTestCases
Change-Id: I784d107085721ad0dabe94bdab33e9cc073f98e4

libwvdrmengine/include/wv_metrics.h

@@ -7,13 +7,14 @@
 #ifndef WV_METRICS_H_
 #define WV_METRICS_H_
 
-#include <string>
+#include <ostream>
 
 #include "wv_metrics.pb.h"
 
 namespace wv_metrics {
 void FormatWvCdmMetrics(const drm_metrics::WvCdmMetrics& metrics,
-                        std::string& result);
+                        size_t parent_indent,
+                        std::ostream& os);
 }
 
 #endif  // WV_METRICS_H_