Fix focus and announcements for Magnification Settings

1. When opening the settings panel in Talkback, the focus will be on the
   panel.
2. Add content description for the magnification settings panel.
3. Fix the click action announced for the magnification window to be
   opening the settings panel.

Bug: 241996711
Test: manually - attach videos to the bug
Test: atest WindowMagnificationSettingsTest
Change-Id: If787cf8b8ce270a5048c650d26905a6123f838ca