Merge Google and Browser sources and call them Web

This required a gigantic refactoring of QuickSearchBox to work nicely.

This change also reduces the number of promoted source to 3,
since Web is now one source instead of two.

As a side effect of the recatoring, VoiceSearch now searches
the selected corpus (fixes http://b/issue?id=2438309)

Fixes http://b/issue?id=2365770

Change-Id: Ife8d40ef62ea004e8d0f20a60e9196fc589f01fc
99 files changed