Read process allocation sizes on system ion heap

We are planning to use this to attribute low-memory kills to processes.

Assume that each represents an allocation. Compute values for all fields
in the ProcessSystemIonHeapSize atom.

This code is intented to run only on userdebug or eng builds and it has
to be enabled via a statsd metric from the server only for devices with
required SELinux policies.

It extends SystemIonHeapSize atom to provide per-process ion allocations
attribution.

Bug: 130526489
Test: atest MemoryStatUtilTest
Test: statsd_testdrive returns non-zero values
Change-Id: I1c1f31e2739dc493e8f4b3f47d73f5bbaadc8bdf