Improve the way mathematical formulas are exposed to screen readers.

- Mark the png and svg fallback as aria-hidden="true".
- For browsers without good enough MathML visual rendering, 
  hide the <math> tag in a way that still makes it accessible to 
  screen readers.

Bug: 66537
Change-Id: I7ecf07a4da669d927e11265fd497a813786c6410