Reposition dropdowns when they would go offscreen (for iPad)
Reposition dropdowns when they would go offscreen (for iPad)

Summary:
The problem:
Some especially long dropdowns go off-screen when viewed on landscape-mode iPad.

The solution:
Check to see if the list will go off screen, and shift it up so that everything fits.

Test Plan:
Setup the iPad app to [use your local version of perseus](https://github.com/Khan/iOS/blob/2944ca85a30c5ae772db283ae395158e4c5098b8/Resources/webview/javascript/perseus-package/perseus-common.js#L198), build for iPad, and go to the "Comparing decimal place value" exercise, in landscape mode. Open the second dropdown, and see that it is shifted up, making all the options selectable. Switch to portait mode and verify that the dropdown is back to its normal position when you expand it.

{F187304}

Reviewers: bryan, charlie, mgp

Reviewed By: charlie

Subscribers: charlie

Differential Revision: https://phabricator.khanacademy.org/D21716

ariabuckles/perseus,learningequality/perseus,ariabuckles/perseus,ariabuckles/perseus,ariabuckles/perseus,learningequality/perseus