Implement metrics 'summarizers'

The summarizers collet similar records and bunch them to save space.
So a string of extractor operations that crack the same format audio
will be combined into a single record with a new "count=#" attribute.

Summarizers will sum appropriately. The current example is in the
Player summarizer -- it knows to sum the frame counts and times
(duration and playing) as part of the summarization.

Bug: 36736083
Test: lots of 'dumpsys -summary' manual examination
Change-Id: I325c2d2b25720d384bcc75b73c97e5b3d8fa9731
diff --git a/media/libmedia/include/MediaAnalyticsItem.h b/media/libmedia/include/MediaAnalyticsItem.h
index f050e7f..dc501b2 100644
--- a/media/libmedia/include/MediaAnalyticsItem.h
+++ b/media/libmedia/include/MediaAnalyticsItem.h
@@ -41,6 +41,7 @@
     friend class MediaAnalyticsService;
     friend class IMediaAnalyticsService;
     friend class MediaMetricsJNI;
+    friend class MetricsSummarizer;
 
     public:
 
@@ -231,7 +232,6 @@
         size_t mPropCount;
         size_t mPropSize;
         Prop *mProps;
-
 };
 
 } // namespace android