Visual tweaks to notifications.

  - Smaller right_icons
  - Higher-contrast text colors
  - Dividers between actions
  - Dividers above actions and overflows
  - Consistent 8dp gutter on left of content
  - BigTextStyle should not show line3 unless there is a subtext.
  - Collapse summary ("overflow") text into line3. This is a
    little wild because now the contentText, subText, and
    summaryText all share this spot, but the various
    variables all have different times when they're
    expressed so you have greater control over what shows
    where in the 1U and the expanded form.
  - Do not show contentText in BigText's line3. If you have
    subtext or summarytext, show that; otherwise suppress
    line3 entirely.

Bug: 6558134 // line3
Bug: 6508804 // other visuals
Change-Id: Ib79e818a332d17000e9a8fce333eff8f8cf043aa