Fix crash when pulling certain atoms

IonMemoryUtil and ProcFsMemoryUtil were package-private classes loaded
in the system server class loader and had the same package as
StatsCompanionService, which is loaded in a separate class loader for
statsd. This caused a crash when statsd tried to access either. This cl
moves IonMemoryUtil and ProcFsMemoryUtil along with StatsPullAtomService
to a separate package so that no classes are shared in the same package.

Bug: 147792532
Test: pulled all relevant atoms and make sure they worked
Test: adb shell cmd stats pull-source 10064
Test: adb shell cmd stats pull-source 10061
Test: adb shell cmd stats pull-source 10042
Test: adb shell cmd stats pull-source 10056
Change-Id: I5107aa47045321e84549a7f2d55d0ee27f0d080e