webauthn: shutdown caBLEv2 when Activity stopped.

onDestroy is called some time after an Activity is dismissed (i.e. "stopped")
in Android terms. But I think caBLE operations should be suspended immediately
when the Activity is dismissed.

BUG=1002262

Change-Id: Idcc09daa17cd2ad691bd26a24c3f4af1ea1a1a6f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2477518Reviewed-by: Martin Kreichgauer <martinkr@google.com>
Commit-Queue: Adam Langley <agl@chromium.org>
Cr-Commit-Position: refs/heads/master@{#817979}

chrome/android/features/cablev2_authenticator/java/src/org/chromium/chrome/browser/webauth/authenticator/CableAuthenticatorUI.java

@@ -173,8 +173,8 @@ public class CableAuthenticatorUI extends Fragment
     }
 
     @Override
-    public void onDestroy() {
-        super.onDestroy();
+    public void onStop() {
+        super.onStop();
         mAuthenticator.close();
     }