[Expressive Battery] Update screen on time text preference style.

For expressive style:
- Update the enlarge font size
- Remove the background

Bug: 349652542
Test: visual
Flag: com.android.settingslib.widget.theme.flags.is_expressive_design_enabled
Change-Id: If85d2903eec6d1f7c3b58e71daef2409325ac83d