Add sign in and sign up with Google to Android samples

[marinacoelho]

This PR adds a new folder to the android video samples folder. It contains the sample code for sign in and sign up with Google using Credential Manager.

Most of the new files are the same from the previous code sample (built for the anonymous authentication video), so focus on the review of:

• build.gradle.kts
• AccountService.kt
• AccountServiceImpl.kt
• AccountCenterButtons.kt
• SignInScreen.kt
• SignInViewModel.kt
• SignUpScreen.kt
• SignUpViewModel.kt

This is how the new UI looks like: