Save last uuid to fix issue with EndCall Transaction

[viniychuk]

The issue is that the link to uuid is not being saved anywhere and than the call to self.callKitCallController requestTransaction:transaction ends up with an error = 4 because it doesn't have the object internally.
Simply saving the link solves the issue (of course it has to be improved when you do the group calls and on-hold calls, but it does the job for now)

[ianlin]

Hi @viniychuk, thanks for the PR. I've got 2 questions:

1. I didn't see you use the saved _lastUUID instance variable. If it's not being used why we are going to save it?
2. Looks like there are some fixes that isn't related to this PR, which are under functions configureAudioSession and performSetHeldCallAction. If they are needed by your side can you please submit a separated PR and elaborate it more?

Cheers!