Fix crash on app start on Android 12 devices

Android 12 requires to specify mutability of the PendingIntent that is
used for the NFC intent.

Choosing a mutable PendingIntent worked out.

https://developer.android.com/about/versions/12/behavior-changes-12#pending-intent-mutability

https://developer.android.com/guide/components/intents-filters#DeclareMutabilityPendingIntent

app/src/main/java/de/mw136/tonuino/nfc/NfcIntentActivity.kt

@@ -31,7 +31,7 @@ abstract class NfcIntentActivity : AppCompatActivity() {
         val intent = Intent(this, javaClass).apply {
             addFlags(Intent.FLAG_ACTIVITY_SINGLE_TOP)
         }
-        pendingIntent = PendingIntent.getActivity(this, 0, intent, 0)
+        pendingIntent = PendingIntent.getActivity(this, 0, intent, PendingIntent.FLAG_MUTABLE)
         nfcAdapter = NfcAdapter.getDefaultAdapter(this)
     }