Expose ImsExternalCallState constructor as @SystemApi

1) Expose the ImsExternalCallState constructor as @SystemApi
so that vendors can use this class correctly.
2) Change the call ID from int->String to comply with RFC 4235.

Bug: 117555643
Bug: 115697579
Test: Manual
Change-Id: I980ec091c69d2b3179843bd8485955377b88712e
2 files changed