Send accessibility event for content change upon setting content description.

1. Since the content description is generated dynamically we need to notify
   clients when it changes so they can drop cached state to get the most
   recent content. This really is used by the caching we do to optimize
   the window query APIs. Otherwise, the user does not see the current
   content.

bug:7327556

Change-Id: I9be46508e86864566e027c64565eb1d787ec9363