Several screensaver picker UX improvements.

1. Remove left/right margin from the screensaver picker grid.
2. Reduce space between icon and title for each screensaver item in the
   screensaver picker grid.

This will make the grid flush (vertically) with the other preferences on
the screen.

Bug: 415834742
Test: manually
Flag: android.service.dreams.dreams_v2
Change-Id: I0c21eb24c1f91321fbd63c89ebca80fc294e9035