Adjust custom theme flow's preview layout

 - Adjust the layout to fit the spec, go/custom-r-guidelines slide#159 &
 160

 video: https://drive.google.com/file/d/1FSx9BQo_JJPhxX6CdKPdpzai8U9LUIEo/view?usp=sharing

 Bug: 160939676
 Test: manually

Change-Id: Iaaf8848f869b9e54e574283762ebb97276483c18