Update CMake files with FIRInstanceId's new layout

Instance ID changed their layout to be inline with the other iOS SDKs, so update the CMake scripts to use that layout instead.

PiperOrigin-RevId: 244440380

Change Functions to be more robust if App is deleted out from under t…

…hem.

If App is deleted, invalidate the internal pointers (e.g. FunctionsInternal)
rather than the outer pointers (Functions), so that later code can still
delete the outer pointer.

PiperOrigin-RevId: 238808270