Improve Screen Reader support for Image Block (selected state)

Unselected state: 
- PR: https://github.com/wordpress-mobile/gutenberg-mobile/pull/794

Selected State: 
- Let select the image with feedback. Read alt if any.
- Access caption field to edit.
- Access Toolbar Buttons.

Settings Bottom Sheet depends on Links UI depends on: https://github.com/wordpress-mobile/gutenberg-mobile/issues/914