Adjust custom theme flow preview page's a11y set

 - Change part of the preview layout to not import for a11y, make it
 won't be focused by TB
 - Add new content description to each preview layout
 video: https://drive.google.com/file/d/1JTZCv7bdRuvzbYbxJXU-yBto4UcivijA/view?usp=sharing

 Fixes: 160760041
 Test: manually

Change-Id: I70df6d74bb39f75641b8200f5ea505d9c21d2b11
3 files changed