speech: Update createSpeechRecognizer javadoc

SpeechRecognizer#createSpeechRecognizer(Context, ComponentName) should
be called only with ComponentNames that are actually mapped in the
running environment.

This update documents how to check the ComponentName is acutally mapped
to RecognitionService.

Bug: 187176179
Test: n/a javadoc change only
Change-Id: Icbf1595f613d6f341b84879122e5eefce6c0fb4a