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

services/core/java/com/android/server/stats/pull/StatsPullAtomService.java

@@ -1468,6 +1468,9 @@ public class StatsPullAtomService extends SystemService {
     }
 
     private void registerIonHeapSize() {
+        if (!new File("/sys/kernel/ion/total_heaps_kb").exists()) {
+            return;
+        }
         int tagId = FrameworkStatsLog.ION_HEAP_SIZE;
         mStatsManager.registerPullAtomCallback(
                 tagId,