Improve summarized statistics

better management of how summarized records are managed.
Use differentiated names for the summations to avoid confusion with
individual records. Track values appropriate to calculating
min, max, mean, and standard deviation.

Bug: 65027360
Test: examination of dumpsys media.metrics -summary
Change-Id: I40d93d8a5a7cc2c188546574f59d557088d9c1e3