Introduce new atom to replace existing ProcessMemoryState and
NativeProcessMemoryState ones for telemetry purposes.
This change is the first step to decouple memory stats from the
framework internals. Follow-up CLs will remove the telemetry parts
from the existing atoms.
Test: atest ProcfsMemoryUtilTest
Test: statsd_testdrive 10064
Change-Id: I7959c6e473201c10e7da17fa49d864bb03a2403c