Added mobile support for Transformer inputs

Summary:
Inputs in the Transformer now support the full mobile InputPaths API for accessing, focusing,
blurring, setting values, etc. It's a lot more code than I would've liked, but then again, these
inputs are very deeply nested within the overall <Transformer> component, so this information has to
get threaded somehow.

I've tested this on mobile, and writing, focusing, blurring, etc. all works. The iOS app crashes when
I use the arrows to navigate between inputs, but this seems like a native bug (and I'll debug it with
Kasra tomorrow).

Test Plan:
- Run Perseus.
- Create a Transformer and enable "Formal" mode.
- Make a bunch of inputs (i.e., hit a few transforms).
- Try printing all the input paths with `itemRenderer.getInputPaths`.
- Try setting the value of an input with `itemRenderer.setInputValue`.
- Try accessing the DOM node, etc.
- Run the mobile app.
- Go to "Defining congruence through rigid transformations" and play around.

Reviewers: jack

Reviewed By: jack

Subscribers: kasra, mgp

Differential Revision: http://phabricator.khanacademy.org/D11792