Add support for schema.org actions in the assist data bundle.

Allow developers to include schema.org metadata to represent an action
a user is performing on the screen.

This will allow applications to report content a user is viewing (ViewAction),
music the user is listening to (ListenAction), etc.

Design: http://go/appindexingapi:bundle

Change-Id: I34544880ac877ffe44671139585304f0498a64eb