Variety of small UX/Design tweaks.

Search field's clear "x" button larger.

Search field reverts to "Search Wikipedia" after
5 seconds, but when you tap on search field again
it remembers your last search term.

Lightened search field's "Search Wikipedia" text
color.

Change-Id: I97491fd962ed8637253440b5fc47afc9c31ebc08
Merge "Variety of small UX/Design tweaks."