Move RSS HWM stats reading code under stats

The RSS high-water mark stats are used only by telemetry and should not
be a part of am package.

Modifications:
- avoid null, use empty string instead
- use assertThat from Truth

Bug: 140986627
Test: atest MemoryStatUtilTest
Test: atest ProcfsMemoryUtilTest
Test: statsd_testdrive for RSS HWM atom
Change-Id: I60140c085f1a47d71505955d79f545f52a4c4423