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