Register IonHeapSize puller only if supported

IonHeapSize atom can be created only if kernel exposes a sysfs node with
the information, otherwise 0 is returned.

Register the puller conditionally only if the sysfs node is present.
This avoids the overhead of creating the atom and simplifies the
server-side configuration.

Bug: 15044292
Test: atest UidAtomTests#testIonHeapSize_enabled
Test: atest UidAtomTests#testIonHeapSize_disabled
Change-Id: Idcbd2564d5fa087dd0a936ab55f02271741c1462