don't allow updating system packages to the same versionCode

versionCode of many system packages, including privileged ones, is set to the current SDK version
and is thus not incremented during non-major OS upgrades.
This allowed to downgrade them to the older version that had the same versionCode.

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

@@ -1595,7 +1595,22 @@ && cannotInstallWithBadPermissionGroups(parsedPackage)) {
                 }
 
                 if (abortInstall) {
-                    throw new PrepareFailure(INSTALL_FAILED_INTERNAL_ERROR, message);
+                    throw new PrepareFailure(PackageManager.INSTALL_FAILED_INTERNAL_ERROR, message);
+                }
+            }
+
+            if (parsedPackage.getLongVersionCode() == systemPackage.getLongVersionCode()) {
+                String message = "Not allowed to update system package to the same versionCode";
+                boolean abortInstall = true;
+
+                if (Build.IS_DEBUGGABLE) {
+                    if (SystemProperties.getBoolean("persist.disable_same_versionCode_sys_pkg_update_check", false)) {
+                        Slog.d(TAG, message + ": " + parsedPackage.getManifestPackageName());
+                        abortInstall = false;
+                    }
+                }
+                if (abortInstall) {
+                    throw new PrepareFailure(PackageManager.INSTALL_FAILED_INTERNAL_ERROR, message);
                 }
             }
         }