Wrap use of AppInternal in INTERNAL_EXPERIMENTAL in public app header file.
Wrap use of AppInternal in INTERNAL_EXPERIMENTAL in public app header file.

PiperOrigin-RevId: 265580216

firebase/firebase-cpp-sdk,firebase/firebase-cpp-sdk,firebase/firebase-cpp-sdk,firebase/firebase-cpp-sdk,firebase/firebase-cpp-sdk,firebase/firebase-cpp-sdk,firebase/firebase-cpp-sdk,firebase/firebase-cpp-sdk