Fix matrix mobile styles

Summary:
Fix the matrix widget styles on mobile. Michelle figured out
the styles and the dimensions, I fixed up the code and fixed the input
path bug.

Test Plan:
 - Make a perseus exercise locally (not with mobile styles)
  - Ensure that everything still works and looks good
 - Make a perseus exercise on a local webapp, visit it in mobile
  - Ensure that the brackets look good
  - Ensure that the brackets move when inputs are selected
  - Ensure that if a large expression is written, it is clipped and
    doesn't make the whole thing look bad

Reviewers: alex

Reviewed By: alex

Subscribers: aria

Differential Revision: https://phabricator.khanacademy.org/D15329