Switch from GCM to FCM.

src/org/thoughtcrime/securesms/gcm/FcmUtil.java

@@ -0,0 +1,43 @@
+package org.thoughtcrime.securesms.gcm;
+
+import android.support.annotation.WorkerThread;
+
+import com.google.firebase.iid.FirebaseInstanceId;
+
+import org.thoughtcrime.securesms.logging.Log;
+import org.whispersystems.libsignal.util.guava.Optional;
+
+import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.atomic.AtomicReference;
+
+public final class FcmUtil {
+
+  private static final String TAG = FcmUtil.class.getSimpleName();
+
+  /**
+   * Retrieves the current FCM token. If one isn't available, it'll be generated.
+   */
+  @WorkerThread
+  public static Optional<String> getToken() {
+    CountDownLatch          latch = new CountDownLatch(1);
+    AtomicReference<String> token = new AtomicReference<>(null);
+
+    FirebaseInstanceId.getInstance().getInstanceId().addOnCompleteListener(task -> {
+      if (task.isSuccessful() && task.getResult() != null) {
+        token.set(task.getResult().getToken());
+      } else {
+        Log.w(TAG, "Failed to get the token.", task.getException());
+      }
+
+      latch.countDown();
+    });
+
+    try {
+      latch.await();
+    } catch (InterruptedException e) {
+      Log.w(TAG, "Was interrupted while waiting for the token.");
+    }
+
+    return Optional.fromNullable(token.get());
+  }
+}