speech: Null-check service connection before usage

In SpeechRecognizer handleStartListening, handleStopMessage and
handleCancelMessage, there is a null-check before calling
the service’s methods. The same check is now added to
handleCheckRecognitionSupport and handleTriggerModelDownload.

Bug: 282013412
Test: atest CtsVoiceRecognitionTestCases
Change-Id: I506282779e02747f8904cf9cbbdbc3ce135ee2b7