Announce the existence of magnification settings

We could not put focus on the magnification settings automatically since
it is an inactive window. Therefore, we announce the content description
of magnification settings when the panel is opened to allow users to be
aware of the existence of it.

Bug: 274734936
Bug: 241996711
Test: manually - attach videos to the bug
Change-Id: I99c2865befa92f0026e57776f288a878cd3a8253