Document training app feature for android flavors

[abbyad]

For medic/cht-android#258

[latin-panda]

LGTM!

[ngaruko]

Tested with unbranded but switching on and off the IS_TRAINING_APP flag

unbranded {
      // we will not create project-specific src directories
      // for `unbranded` - it will use the defaults in
      // src/main
      dimension = 'brand'
      buildConfigField "boolean", "IS_TRAINING_APP", 'true'
    }

When the flag is set to true, a toast shows at the beginning of the app

and an orange contour around all screens

And none when the flag is set to 'false'

Good to merge @abbyad .