[DocsUI M3] Update styles for skeleton and app bar

1. Rename some dimension variable to disambiguate padding/margin.
2. Add padding end for app bar, update padding start for drawer
   layout to align the menu button with the filter bar.
3. Move the padding from the main container level (right section)
   to the main layout to match the new spec. This will make sure
   the scroll area of the nav tree has the same padding top as
   the right section.

Check the bug item for before/after screenshots.

Bug: 377771946
Test: Manually inspect after deployment
Flag: com.android.documentsui.flags.use_material3
Change-Id: I3eece6848731dc811792ca18d5a898ac26090034