Refine the preference layout for homepage

Based on the Android S design, we update the layout
for homepage.

The scope covers the layout of preference and icons itself.

- Change the icon size for IA items.
- Change the layout spacing of preference.
- Change the margin or search bar.

Test: See the screen and confirm with UX
Bug: 177213699
Change-Id: I985966fdf39a5989d51991aa3c06ea5fd31e8fe2