oauth2_http: Use Futures.getDone() instead of Future.get() in listener (#1109)

Listeners for Futures should generally use Futures.addCallback() or
Futures.getDone() when called. This makes it clear they won't block and
allows tooling to know it will never block.

cl/492555987

Co-authored-by: Timur Sadykov <stim@google.com>

Author: ejona86

oauth2_http/java/com/google/auth/oauth2/OAuth2Credentials.java

@@ -284,12 +284,10 @@ public OAuthValue call() throws Exception {
   private void finishRefreshAsync(ListenableFuture<OAuthValue> finishedTask) {
     synchronized (lock) {
       try {
-        this.value = finishedTask.get();
+        this.value = Futures.getDone(finishedTask);
         for (CredentialsChangedListener listener : changeListeners) {
           listener.onChanged(this);
         }
-      } catch (InterruptedException e) {
-        Thread.currentThread().interrupt();
       } catch (Exception e) {
         // noop
       } finally {