Fix Number-line label bugs

Summary:
Fix two bugs:
(1) label drawing over point during tick ctrl transitions
(2) label duplicated, and drawing in slightly different locations

I attempted the toFront route, but evidentally labels don't have that function, so this will have to do.

Test Plan:
(1) Make the range 0 to 2, put labels at 0 and 5/3, enable tick controller
(2) Ensure that moving the tick controller doesn't make it look messed up

Reviewers: jack

Reviewed By: jack

Subscribers: alpert

Differential Revision: http://phabricator.khanacademy.org/D7438