gradle: squashed maintainance

 - upgrade wrapper to 7.0.2
 - upgrade plugin to 4.2.1
 - use mavenCentral instead of jcenter
  - Jcenter is end of life.

Change-Id: Ia962fed30fb62c82ba7fb5d2c8079dd3ccac58bd
Signed-off-by: Alexander Martinz <amartinz@shiftphones.com>