Add ProcessMemorySnapshot atom

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