Cannot Reassign Keyboard and Mouse Controls

When selecting the keyboard and mouse controls in the settings, the controls cannot be reassigned. (Tested on Android, presumably affects other mobile devices too)

Steps to Reproduce:

• Go to Settings > Keyboard and Mouse
• Try to reassign any of the controls
• Tapping a key or mouse button does nothing

NB: This doesn't seem to affect controllers, and the keys can be reassigned on W10.

Tested on OnePlus One A0001 on Android 6.0.1 (CM13) and Samsung TabPro SM-T520 on Android 4.4.2