Move ion stats reading code under stats

The ion 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: 140788538
Test: atest MemoryStatUtilTest
Test: atest IonMemoryUtilTest
Test: statsd_testdrive for ion atoms
Change-Id: Id31f98e3b3ea39923f599f89e0f71b9293b3eb92