Share permissions code with medic-android

[alxndrsn]

Ref medic/cht-android#43

Some notes:
isDemand and X_IS_DEMAND are poorly named. They appear to be used as a final option when the user has told the app to stop asking_, rather than for permissions which are demanded rather than requested.

There is no differentiation between permissions which are required, and permissions which are optional but helpful to have. OTOH this probably isn't a distinction we care about right now.