Public API for RemoteControlClient ratings and editable metadata

A rating for RemoteControlClient content is:
  - a value between 0 and 100
  - or a value indicating there is no rating
 For a same piece of content, a rating can come from:
  - the user
  - "others" (i.e. not the user), to provide an average rating
 Rating styles are:
  - heart (a toggle)
  - thumb up / down
  - stars (with a configurable maximum number of stars)

For editable metadata, add:
  - methods to control which keys can be edited,
  - interface for an application to receive new values for a key.

Rating by user is editable metadata.

Bug 8440498

Change-Id: Ia9f61e750772658051cea1ac7c316187717f0f58