Introduce IonHeapSize atom

IonHeapSize is replacing SystemIonHeapSize on devices with newer
kernels. It relies on libmeminfo to measure the total size of all ion
heaps, unlike the old atom that manually parsed a file from debugfs.

Test: m -j
Test: statsd_testdrive 10070
Bug: 140788538
Bug: 147722878
Change-Id: If3fa74bd96631116ef05656e7942fa6cf4e15005