Add system property to allow testing OTA with the same build image

The property can be used to test post boot stage without needing
to change the build signature.

Test: make
Change-Id: Ic132da7fda346f0cced4fd9417a34f59501ae7ee

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

@@ -3381,7 +3381,9 @@ public class PackageManagerService extends IPackageManager.Stub
     @Override
     public boolean isUpgrade() {
         // allow instant applications
-        return mIsUpgrade;
+        // The system property allows testing ota flow when upgraded to the same image.
+        return mIsUpgrade || SystemProperties.getBoolean(
+                "persist.pm.mock-upgrade", false /* default */);
     }
 
     private @Nullable String getRequiredButNotReallyRequiredVerifierLPr() {