[DocsUI M3] Uplift picker/save fragment spacing

Update the spacing for the picker and save fragments to match
the figma specs. Previously the padding distributed between an outer
and inner container which was confusing. Instead just have padding for
one container layer and margins for the buttons/items. Also remove an
unnecessary outer container in the fragment_save.xml.

Container top/left/right padding:
- small1 (compact, medium, expanded)

Container bottom padding:
- small1 (compact) (Actually this should be 0dp if there is a gesture
  navigation bar, address this in b/402604097)
- 0dp (medium, expanded)

Space between horizontal buttons/items:
- extraSmall4 (8dp) (compact, medium, expanded). So set
  the margin for each button to be 8/2 = 4dp (extraSmall2)
  when it is adjacent to another button. Do not set start/end margins
  on the start/end buttons.
- to handle spacing of the progress indicator in the save fragment,
  that appears after the last button (Save) only when there is a pending
  save, set a 0 end margin for the Save button and set the start margin
  of the progress indicator to be extraSmall4.

Buttons top and bottom padding:
- extraSmall2 (compact, medium, expanded)

See bug for screenshots.

Bug: 412550239
Test: m DocumentsUIGoogle && manual inspection
Flag: com.android.documentsui.flags.use_material3
Change-Id: I69f4d5b16120b226d5e62283e784990ebffc2a0e