Fix SpeechAnnouncementListener example and add tests

[danesfeder]

For the example of SpeechInstructionListener in EmbeddedNavigationActivity, we were modifying a SpeechAnnouncement created with a VoiceInstructionMilestone (this will always be the case).

Because it was create with the milestone, the milestone data was overriding the modification. I added some javadoc to clearly state that this will happen as well.

[Guardiola31337]

Test that AutoValue works as expected is not worth it IMO. Ultimately, it's like testing a setter. 👀 https://speakerdeck.com/guardiola31337/elegant-unit-testing-droidcon-berlin-2016?slide=68

[danesfeder]

@Guardiola31337 this is testing custom build logic within the AutoValue class

mapbox-navigation-android/libandroid-navigation-ui/src/main/java/com/mapbox/services/android/navigation/ui/v5/voice/SpeechAnnouncement.java

Lines 101 to 111 in 13685e8

	private SpeechAnnouncement buildSpeechAnnouncement() {
	VoiceInstructionMilestone milestone = voiceInstructionMilestone();
	if (milestone != null) {
	ssmlAnnouncement(milestone.getSsmlAnnouncement());
	announcement(milestone.getAnnouncement());
	return autoBuild();
	} else {
	return autoBuild();
	}
	}
	}

Still not worth it you think?