Add a class to represent the set of sensitive features of a web application

This class is used to store the set of sensitive features for which the web app
is authorised. The data for which is passed as meta data with the intent. The
data is then used to set the security policy for features such as Geolocation
which request permission through the browser chrome.

Bug: 2766920
Change-Id: I2ff259dfd3b50d39bb05fbc187d4943c448d08d6