refactor(database): use Jetpack's Navigation Component

[thatfiredev]

Part of #1217, this should implement the Navigation Component in our database quickstart.

Extra changes

In order to accommodate the Navigation implementation, some extra changes took place here:

• deleted the NewPostTest unit test class which was testing one of our activities that no longer exists.
• edited the AppTheme.NoActionBar theme because it was causing the systembar to disappear.
• the fragment package which contains the fragments used in a TabLayout was renamed to listfragments to avoid any confusion.
• moved the CommentViewHolder out of the PostDetailActivity class and into a separate class in the viewholder package (now sitting pretty alongside its brother PostViewHolder).

Snippets moved to firebase/snippets-android

See firebase/snippets-android#275

@samtstern there are some snippets that are present in this repo, but I can't find them in the docs.
For example, the my_top_posts_query, which is similar to another snippet already present in snippets-android:

quickstart-android

quickstart-android/database/app/src/main/java/com/google/firebase/quickstart/database/java/fragment/MyTopPostsFragment.java

Lines 12 to 17 in 49bcffe

	// [START my_top_posts_query]
	// My top posts by number of stars
	String myUserId = getUid();
	Query myTopPostsQuery = databaseReference.child("user-posts").child(myUserId)
	.orderByChild("starCount");
	// [END my_top_posts_query]

snippets-android

https://github.com/firebase/snippets-android/blob/45ba4822eb111609be2951c944142e3bbe891472/database/app/src/main/java/com/google/firebase/referencecode/database/QueryActivity.java#L98-L113

I can see its usage in the docs: https://firebase.google.com/docs/database/android/lists-of-data#sort_data

Snippets in a similar situation include: comment_class, single_value_read and declare_database_ref. Is it possible to check these against the snippet registry?

[samtstern]

@rosariopfernandes thank you so much for being so careful about snippets as you do this. That's really helpful.

So here's what we'll do:

1. Merge your snippets-android change
2. Make the docs point to the new snippets (internal change)
3. Merge this one.

[samtstern]

This one is ready to go! Internal change is in so this should be safe.