Ensure notification color contrast

Ensure we have at least 3:1 contrast for the
action buttons. Also ensures that the inline
reply box background has 4.5:1 contrast to white.

Further modifies the color of the inline reply
background to satisfy a 4.5:1 contrast ratio for
the entered text.

Bug: 26831312
Change-Id: If42b1c99d1adee547d0a583c1a69c48ef7287c23