Add ion/ion_stat to the atrace memory category

ion_stat was added to 4.19+ kernels as a successor to the existing
ion_heap_grow / shrink tracepoints

Bug: 154302786
Test: cuttlefish with new kernel, run atrace memory and cat /sys/kernel/tracing/events/ion/ion_stat/enable
Change-Id: I2502d281be0a1de5a0bde41482e70ad8ef29fd0b