Update Launcher to use theme colors.

Instead of using hard-coded variants of colors, use the theme color
that corresponds (colorAccent, colorPrimary, etc).

Updated:
- "Search for more apps" text
- App info drop target hover color
- PageIndicatorDots
- All apps & widget scroll bar & scroll popup colors
- All apps section text color
- widget picker background color

TODO:
- widget picker header color

BUG: 28625102
Change-Id: I74b543ff71b8caef8cd950c8183ac84c759cb4d5