Adds espresso testing for add_to_app/android_fullscreen

[RedBrogdon]

• Adds the espresso plugin to flutter_module as a dev dependency.
• Updates android_fullscreen to use espresso testing
  • Adds a debug manifest that turns cleartext traffic on (req. by espresso plugin)
  • Adds two new android tests using the plugin's espresso tech
  • Adds multidex support
• Updates travis script for Android to invoke the new tests via Firebase Test Lab
  • Travis's emulators will not work for the espresso tests.

[RedBrogdon]

This is so we don't accidentally commit the credentials file to the repo.