Use simpler syntax for Google Maven repository

Summary: This follows the guidelines described at https://developer.android.com/studio/build/gradle-plugin-3-0-0-migration.html#apply_plugin

Test Plan: Try to build with this change (./gradlew assembleRelease or similar)

Reviewers: KDE Connect, mtijink

Reviewed By: KDE Connect, mtijink

Differential Revision: https://phabricator.kde.org/D11645