Fix #addChildSessionId (2/n)

A multi-session can't be a child.

Bug: 162286562
Test: Will be added in next CLs
Change-Id: I7e9957f0cd57fe924cfb5ba8f0cfb27b045f26dc

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

@@ -3101,6 +3101,10 @@ public class PackageInstallerSession extends IPackageInstallerSession.Stub {
             throw new IllegalStateException("Unable to add child session " + childSessionId
                     + " as it does not exist.");
         }
+        if (childSession.params.isMultiPackage) {
+            throw new IllegalStateException("Multi-session " + childSessionId
+                    + " can't be a child.");
+        }
 
         try {
             acquireTransactionLock();