reduce code duplication between onionmasq-mobile and onionmasq-apple

while grepping around to investigate some other issue, i quickly noticed that there is much duplication between these crates. This is bound to make both module diverge, leading to harder maintenance, and bug fixes implemented only one one side.

(onionmasq-mobile could also use a rename to onionmasq-android as it isn't "everything mobile", as the existence of onionmasq-apple shows)