docs: Adding Udacity link to Android TV training page

Updating Android TV training index page to include link and
thumbnail of Udacity Android TV training class video as
per this request:

https://docs.google.com/document/d/1PiJj4ZnAjW41oanl_NRlQNMHQLpyyg2SWt43WcWsbds/edit

Bug: 25398575
Change-Id: I187909ac7b23a85f99c5cf39bbf3cf0687ab2319