iOS: FingerId may not be stable

[madsmtm]

I think it may be prudent to use touch.hash() (importing NSObjectProtocol) instead of just converting to a pointer and comparing the address? I'm not entirely sure that the ID will be consistent otherwise, UITouch seems to be an immutable container that also contains e.g. the location of the current touch (so it'll probably change next time we process the specific touch).

Firefox' code seems to rely on inserting the UITouch into a hashmap, to store a stable identifier, so perhaps that's actually what we need to do?

Found while reviewing #3947.