Remove callbacks from Ava.
Remove callbacks from Ava.

1. Use promises and return observable instead.

Bug: 117574672
Change-Id: If43633ba83156581c6c8962869a7bdc0285d7f5d

actions-on-google/actions-on-google-testing-nodejs,actions-on-google/actions-on-google-testing-nodejs,actions-on-google/actions-on-google-testing-nodejs