input-number: call focus on click

Summary:
This should resolve an issue mobile was having where readonly inputs
would not focus.

Test Plan:
Make sure that inputs still behave normally and send only one onFocus
event when clicked in a web page on a computer
Send to laura and see that input focusing and unfocusing
(losing the focused state) now works

Reviewers: alpert

Reviewed By: alpert

Subscribers: laura

Differential Revision: http://phabricator.khanacademy.org/D9846