[FlightRPC][Java] CallbackBackpressureStrategy should not rely on listener.isReady()

According to the spec for , we can get into a state where the ready flag on the listener has already switched back to false by the time we execute the on ready handler.

We currently use the listener's ready flag after the ready handler to check if we received a spurious wake, but it's not reliable because of this. Instead we should use our own readiness flag that gets set within a synchronized block local to the CallbackBackpressureStrategy.

**Reporter**: [James Duong](https://issues.apache.org/jira/browse/ARROW-15876) / @jduo

_{**Note**: *This issue was originally created as [ARROW-15876](https://issues.apache.org/jira/browse/ARROW-15876). Please see the [migration documentation](https://github.com/apache/arrow/issues/14542) for further details.*}

Beginne mit CallbackBackpressureStrategy und der im Issue verlinkten CallStreamObserver setOnReadyHandler-Spezifikation. Verfolge, wie das Bereitschafts-Flag des Listeners nach dem Ready-Handler geprüft wird, und verifiziere anschließend, dass der Abschluss nicht mehr von diesem Listener-Flag abhängt und den synchronisierten Bereitschaftszustand der Strategie verwendet.

Vom Indexierungsmodell aus dem Issue-Text verfasst.