Fix #addChildSessionId (1/n)

A single-session can't have child.

Bug: 162286562
Test: Will be added in next CLs
Change-Id: I17037a6a65599a74cffd12fffc741b924143dc1a

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

@@ -3092,6 +3092,10 @@ public class PackageInstallerSession extends IPackageInstallerSession.Stub {
 
     @Override
     public void addChildSessionId(int childSessionId) {
+        if (!params.isMultiPackage) {
+            throw new IllegalStateException("Single-session " + sessionId + " can't have child.");
+        }
+
         final PackageInstallerSession childSession = mSessionProvider.getSession(childSessionId);
         if (childSession == null) {
             throw new IllegalStateException("Unable to add child session " + childSessionId