Tweak uber search dropdown styles

Summary: Moved the dropdown outside of the input to give text more space. Also changed the 'current app' icon to the target icon, it feels a little better in use. I will follow up with a fix to T5218.

Test Plan:
Tested desktop, mobile, tablet searching.

{F379466}

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D12516