statsd pull procstats data

Mimic the behavior of existing dumpsys.
We maintain a high water mark for procstats collection.
Only stats in procstats proto output will be collected.
Currently it is about 100kB in size, as compared to 200kB with text
dump.

Bug: 113075306
Bug: 113075820
Bug: 116168047
Test: cts test
Change-Id: Ica9f31bf1ef447e1dc5a24c3cd3ab97c17e33b63