replace jcenter() with mavenCentral()

[thatfiredev]

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

codelab-friendlyeats-flutter/android/build.gradle

Lines 5 to 18 in 6f7603c

	jcenter()
	}
	dependencies {
	classpath 'com.android.tools.build:gradle:4.0.1'
	classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
	classpath 'com.google.gms:google-services:4.3.3'
	}
	}
	allprojects {
	repositories {
	google()
	jcenter()

The approach we've been taking on Android repos is to replace it with mavenCentral(): firebase/snippets-android@5b9f209
I think it would be nice to do it here too.

[ditman]

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