Update hintsUsed just for building the hint network request

Summary:
Addresses the hint count logging bug pointed out in
rKE0732bbbdfa6f04768ac9ea37f85343beacc862fd
while still updating hintsUsed in the showHint event rather than
on the button press, in case hints are triggered outside of
the hint button.

Basically, the problem was that the request we send to the server
to let it know that a hint was taken takes the current number of
hints, which didn't include the hint we are requesting itself.
This seems to cause the hint numbers to be off by one in the logs.

Test Plan:
~~#yolo~~

(Stolen from rKE0732bbbdfa6f04768ac9ea37f85343beacc862fd)

Open up devserver, make new user, skip pretest, do a problem. Wait ~5 seconds, get hint. Wait ~5 seconds, attempt problem. Wait ~10 seconds, get hint. Wait ~15 seconds, get hint.
Verify that the correct hint #'s are being displayed.

Run devshell.py and get the problem log:
user = UserData.get_from_auth_email('atest@example.com')
problems = ProblemLog.get_for_user_data_between_dts(user,datetime.datetime(2014,1,1),datetime.datetime(2014,12,31))
problems[0].hint_after_attempt_list
problems[0].hint_time_taken_list

Prior to the change, the hint_after_attempt_list would be [0,1] and hint_time_taken_list would be [5,15]
After the change, the hint_after_attempt_list should be [0,1,1] and hint_time_taken_list should be [5,10,15]

Reviewers: eater

Reviewed By: eater

Subscribers: lauren

Differential Revision: http://phabricator.khanacademy.org/D12062