MDC-103: Wrong app bar color in Android 10

[marcelpinto]

Describe the bug
The color of the AppBar is grey but in the tutorial is shown as purple

To Reproduce
Steps to reproduce the behavior:

1. Clone the repo for MDC-103 branch
2. Run the code
3. Pass the login screen
4. See grey color in AppBar

Expected behavior
The color should be purple as the tutorial shows

Screenshots

Codelab details (please complete the following information about the codelab):

• Level MDC-103
• Language Kotlin
• https://codelabs.developers.google.com/codelabs/mdc-103-kotlin/#1

Additional context
The colorPrimary is the one having the purple but it should be the colorAccent, as later this is the one that is indicated that needs to be changed.