Remove assertion when current activity is null

Summary:
Assertion failure would lead to a crash, in real world not being able to start an activity shouldn't crash - usually a navigation won't happen as expected, user could try again.

Changelog:
[Android][Changed] - Don't assert on current activity when call startActivityForResult

Reviewed By: cortinico

Differential Revision: D35746652

fbshipit-source-id: 0b77ca5a69b2f3f3b0b969d84980ed8290ac9b1f

ReactAndroid/src/main/java/com/facebook/react/bridge/ReactContext.java

@@ -457,9 +457,11 @@ public boolean hasCurrentActivity() {
    */
   public boolean startActivityForResult(Intent intent, int code, Bundle bundle) {
     Activity activity = getCurrentActivity();
-    Assertions.assertNotNull(activity);
-    activity.startActivityForResult(intent, code, bundle);
-    return true;
+    if (activity != null) {
+      activity.startActivityForResult(intent, code, bundle);
+      return true;
+    }
+    return false;
   }
 
   /**