Check if NFC is enabled in onResume activity

This essentially forces the user to enable NFC to even be able to enter
the passport number etc., but you can't really do anything with the app
anyways. So I figured it was better to just forcefully request the user
to enable it?

It shows the help text in a toast message.

Fixes tananaev#49

app/src/main/java/com/tananaev/passportreader/MainActivity.kt

@@ -34,6 +34,7 @@ import android.util.Log
 import android.view.View
 import android.view.WindowManager
 import android.widget.EditText
+import android.widget.Toast
 import androidx.appcompat.app.AppCompatActivity
 import com.google.android.material.snackbar.Snackbar
 import com.tananaev.passportreader.ImageUtil.decodeImage
@@ -164,11 +165,22 @@ abstract class MainActivity : AppCompatActivity() {
         super.onResume()
         val adapter = NfcAdapter.getDefaultAdapter(this)
         if (adapter != null) {
-            val intent = Intent(applicationContext, this.javaClass)
-            intent.flags = Intent.FLAG_ACTIVITY_SINGLE_TOP
-            val pendingIntent = PendingIntent.getActivity(this, 0, intent, PendingIntent.FLAG_MUTABLE)
-            val filter = arrayOf(arrayOf("android.nfc.tech.IsoDep"))
-            adapter.enableForegroundDispatch(this, pendingIntent, null, filter)
+            if (adapter.isEnabled) {
+                val intent = Intent(applicationContext, this.javaClass)
+                intent.flags = Intent.FLAG_ACTIVITY_SINGLE_TOP
+                val pendingIntent =
+                    PendingIntent.getActivity(this, 0, intent, PendingIntent.FLAG_MUTABLE)
+                val filter = arrayOf(arrayOf("android.nfc.tech.IsoDep"))
+                adapter.enableForegroundDispatch(this, pendingIntent, null, filter)
+            } else {
+                Toast.makeText(
+                    getApplicationContext(),
+                    "Please activate NFC and press Back to return to the application!",
+                    Toast.LENGTH_LONG
+                ).show();
+                Thread.sleep(5_000)
+                startActivity(Intent(android.provider.Settings.ACTION_NFC_SETTINGS));
+            }
         }
         if (passportNumberFromIntent) {
             // When the passport number field is populated from the caller, we hide the