Add retry hint to key exchange errors and cancel button to flash dialog

1. Append 'probeer het opnieuw' to key exchange error toasts.
2. Add 'Annuleren' button to flash progress dialog that calls
   flasher.cancel() and cleanupAction().

android/app/src/main/java/com/eddy/atomrelay/SettingsActivity.kt

@@ -169,11 +169,11 @@ class SettingsActivity : AppCompatActivity() {
                         finish()
                     }
                     is KeyResult.Error -> {
-                        Toast.makeText(this, result.message, Toast.LENGTH_LONG).show()
+                        Toast.makeText(this, "${result.message} — probeer het opnieuw", Toast.LENGTH_LONG).show()
                         cleanupAction()
                     }
                     null -> {
-                        Toast.makeText(this, "Sleutel niet gevonden", Toast.LENGTH_SHORT).show()
+                        Toast.makeText(this, "Sleutel niet gevonden — probeer het opnieuw", Toast.LENGTH_SHORT).show()
                         cleanupAction()
                     }
                 }
@@ -287,6 +287,10 @@ class SettingsActivity : AppCompatActivity() {
         progressDialog = AlertDialog.Builder(this)
             .setTitle(title)
             .setView(view)
+            .setNegativeButton("Annuleren") { _, _ ->
+                flasher?.cancel()
+                cleanupAction()
+            }
             .setCancelable(false)
             .show()
     }