Improve reading proc stats

StatsCompanion can now do the process filtering on its own
(e.g. so that it can do prefix matching instead of exact names in future
CLs).

As part of this, change reading cmdline and status to the native reader which
has better performance.

10064 pull timing before: avg: 41ms max: 55ms
                  after:  avg: 35ms max: 41ms

Change-Id: I3fcdffb727b12a98c10e0712272cc3edf0b24208
Test: Manual, atest android.cts.statsd.atom.UidAtomTests