Add content description for preview card of Theme and Grid

Talkback would read "Style preview, double-tap to activate" for theme small preview, and read "Style preview" for theme full preview. And also grid preview.
Video: https://drive.google.com/file/d/11meCX_GTjkDQokJQv9abwfsuGRSL6vpU/view?usp=sharing

Test: Manually
Bug: 157111975
Change-Id: Ieaafad0b6ccc1102bb127f692f6af6b72bc4918a