change button order of verification-broken alert (#2621)

Having "OK" in the middle of the buttons is weird. The old order was:

```
End-to-end encryption cannot...
[More Info]  [OK] [Scan QR code]
```

1. _If_ the main purpose of the dialog would be to "Scan QR code",
   the text should point that out more, also the opening button.
   Then, the button position would be correct, however,
   it has to read "Cancel" then.

2. _However_, the main purpose of the dialog is to "Show Info",
   "Scan QR code" is only an optional offering and the button a shortcut only.
   And finishing "Show Info" is better done by a simple "OK".
   Then, the "OK" should be rightmost.

This PR implements the second option, the buttons read as follows afterwards:

```
End-to-end encryption cannot...
[More Info]  [Scan QR code] [OK]
```

src/org/thoughtcrime/securesms/connect/DcHelper.java

@@ -446,8 +446,8 @@ public class DcHelper {
     new AlertDialog.Builder(context)
             .setMessage(context.getString(R.string.chat_protection_broken_explanation, name))
             .setNeutralButton(R.string.learn_more, (d, w) -> IntentUtils.showBrowserIntent(context, "https://staging.delta.chat/684/en/help#verificationbroken"))
-            .setPositiveButton(R.string.qrscan_title, (d, w) -> context.startActivity(new Intent(context, QrActivity.class)))
-            .setNegativeButton(R.string.ok, (d, w) -> {})
+            .setNegativeButton(R.string.qrscan_title, (d, w) -> context.startActivity(new Intent(context, QrActivity.class)))
+            .setPositiveButton(R.string.ok, null)
             .setCancelable(true)
             .show();
   }