Reject a multi-sessin set as APEX (1/n)

Prevent the crash loop in b/163288861#comment1.

It doesn't make sense to set a multi-session to APEX at all.
We should check and reject this case.

Bug: 163288861
Test: N/A, will be added in next CL

Change-Id: I23147837a11733b776b1147797b545b539c6c05a

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

@@ -613,6 +613,9 @@ public class PackageInstallerService extends IPackageInstaller.Stub implements
                 throw new IllegalArgumentException(
                     "APEX files can only be installed as part of a staged session.");
             }
+            if (params.isMultiPackage) {
+                throw new IllegalArgumentException("A multi-session can't be set as APEX.");
+            }
         }
 
         if (params.isStaged && !isCalledBySystemOrShell(callingUid)) {