public @Nullable CustomTabsSession newSession(final CustomTabsCallback callback) {
        return newSessionInternal(callback, null);
    }
Creates a new session through an ICustomTabsService with the optional callback. This session
can be used to associate any related communication through the service with an intent and
then later with a Custom Tab. The client can then send later service calls or intents to
through same session-intent-Custom Tab association.
@param callback The callback through which the client will receive updates about the created
                session. Can be null. All the callbacks will be received on the UI thread.
@return The session object that was created as a result of the transaction. The client can
        use this to relay session specific calls.
        Null on error.
customtabs/src/android/support/customtabs/CustomTabsClient.java