Allow :provider => :android to also be passed as option "provider" => "android" String pair.

This allows the UrbanAirship gem to be used inside a Resque/Sidekiq job
which will silently convert the options Hash with Symbol values to
String values when the job args are serialized through Redis.

Without this, when we passed arguments to a Sidekiq job to indicate that the job should be processed in the context of an Android request it would fail due to the UrbanAirship gem was very strict in only accepting symbols.  This was a tricky to diagnose issue which would cause registration/unregister calls to fail since the iOS API was being incorrectly tried for Android tokens.