Migrate android samples from jcenter to mavenCentral

[Jenn Magder]

JCenter has been made readonly https://jfrog.com/blog/into-the-sunset-bintray-jcenter-gocenter-and-chartcenter. This has been a cause of flakes in other repos.

      > Failed to list versions for junit:junit.
         > Unable to load Maven meta-data from https://jcenter.bintray.com/junit/junit/maven-metadata.xml.
            > Could not get resource 'https://jcenter.bintray.com/junit/junit/maven-metadata.xml'.
               > Could not GET 'https://jcenter.bintray.com/junit/junit/maven-metadata.xml'.
                  > org.apache.http.client.ClientProtocolException (no error message)

Migrate samples build.gradle configs from jcenter() to mavenCentral() maven repository.
https://github.com/flutter/plugins/blob/e25698f7e4634c00a76f05dd618e303fc7ac4304/packages/url_launcher/url_launcher/android/build.gradle#L7

buildscript {
    repositories {
        google()
-       jcenter()
+       mavenCentral()
    }

    dependencies {
        classpath 'com.android.tools.build:gradle:3.4.2'
    }
}

allprojects {
    repositories {
        google()
-       jcenter()
+       mavenCentral()
    }
}

See flutter/flutter#82847 and flutter/flutter#78338