Skip assistant fallback set-up when it's disabled

Do not set-up the assistant fallback, when it is disabled, i.e.
config_enableFallbackAssistant configuration flag is set to `false`:
1. when the flag is not set, the fallback object will never be used;
2. the instantiation of the `FallbackAssistant` class from the test
triggers the RuntimeException ("Can't create handler inside thread
Thread[...] that has not called Looper.prepare()"), while the
`TextToSpeechHelper` it creates under the hood uses a deprecated
`Handler` constructor
(https://developer.android.com/reference/android/os/Handler#Handler()).

Bug: 175709867
Test: atest -c CarNotificationUnitTests

Change-Id: If2d0f84e54a6f0143d20c163bf6363e5ad1505ad