Android Google plus Sign out button looks different from sign in button in Google tutorial docs

There is no sign-out button provided by google.

If you would like extra buttons in the same style as google's, you will need to create them yourself.

But it's a lot easier than it sounds, there are many resources provided by Google to show you how to create a button that looks just like a Google button. Take a look over here:

https://developers.google.com/+/branding-guidelines

Here is a little example I made in microsoft paint to show you the kind of stuff you can make!

You can make your button look the same as <com.google.android.gms.common.SignInButton>, adding a predefined style to your button with the xml tag style:

style="@style/FirebaseUI.Button.AccountChooser.GoogleButton"

Note that its a FirebaseUI style and you can put the Button text that you want.