Add stats puller for aggregated procstats

Implement puller code in StatsPullAtomService & ProcessStats that will
output new pre-aggregated proto form of ProcStats data.

Change-Id: Id3d28b0ec135b1b25f3b70a5eb29bec67d71805d
Test: local debugging, see b/152397079
Bug: b/152397079
Bug: b/145203981
Merged-In: Id3d28b0ec135b1b25f3b70a5eb29bec67d71805d
(cherry picked from commit 3b428af57ffc799191caac242b1fb6a88156feec)
4 files changed