Don't abandon child sessions (1/n)

It will throw if abandon() is called on a child session.

Bug: 211944991
Bug: 212665855
Bug: 67862680
Test: to be added
Change-Id: Ib0ba9f3786dda2d3174f3ea8c65d1061a3fcb586

services/core/java/com/android/server/pm/PackageInstallerService.java

@@ -404,7 +404,11 @@ public class PackageInstallerService extends IPackageInstaller.Stub implements
                 if (age >= MAX_SESSION_AGE_ON_LOW_STORAGE_MILLIS) {
                     // Aggressively close old sessions because we are running low on storage
                     // Their staging dirs will be removed too
-                    session.abandon();
+                    PackageInstallerSession root = !session.hasParentSessionId()
+                            ? session : mSessions.get(session.getParentSessionId());
+                    if (!root.isDestroyed()) {
+                        root.abandon();
+                    }
                 } else {
                     // Session is new enough, so it deserves to be kept even on low storage
                     unclaimedStagingDirsOnVolume.remove(session.stageDir);