AlertDialogs: Remove useless calls to `create()`

`AlertDialog.Builder.show()` is equivalent to
`AlertDialogBuilder.create().show()` so the extra call to `create()` can
be removed.
CommentsActivity: Remove constructor

Constructors for Activities are weird... lets move that initialization
to the field declaration.
CommentsActivity: Use `this` over `getBaseContext`

Turns out `getBaseContext()` shouldn't really be used ever according to
various stack overflow posts.
GuideView: Default to super.onActivityResult()

Otherwise lots of strange bugs could crop up with `BaseActivity` not
receiving activity results in GuideViewActivity.
GuideViewActivity: Remove unused method
GuideView: Make menu invalidation clearer

Invalidating the menu is used for a lot more than just updating comment
counts. This makes the reasoning behind calling it much more clear.
Comments API call: Return null on JSON error

The chance of this happening is exceptionally low and logging it won't
help anything. Just return null and have it fail hard at the caller.
ApiEndpoint: Make indentation consistent