replace jcenter() with mavenCentral()

[thatfiredev]

Looks like the android module on this codelab is still using jcenter() which has been recently sunset:

https://github.com/FirebaseExtended/codelab-friendlyeats-flutter/blob/6f7603c33f2ef0f7e37ff1730991e1982327aa6f/android/build.gradle#L5-L18

The approach we've been taking on Android repos is to replace it with mavenCentral(): https://github.com/firebase/snippets-android/commit/5b9f209dc700ba31527735e4a923c845ee2c1807 I think it would be nice to do it here too.

[ditman]

Yes, we've had to do this with other repos!