ControllerInterface: Make the ID assigning code common

This makes the device ID assigning code common to all backends, by
moving it to AddDevice() instead of copy-pasting or replicating
the logic in the backends.

Also, to prepare for hotplugging, instead of relying on a name usage
count, the new ID assigning system always starts from ID 0 and tries
to assign the first ID that is not used.

Source/Core/InputCommon/ControllerInterface/Android/Android.h

@@ -46,7 +46,6 @@ public:
   Touchscreen(int padID);
   ~Touchscreen() {}
   std::string GetName() const;
-  int GetId() const;
   std::string GetSource() const;
 
 private: