Add Firebase App C++ integration test.

Also add Windows and OSX-specific builds to the sample_framework Skylark script, and add
Builder tests for those platforms to each integration test's METADATA.

(Also fixes a small issue in AdMob header when building the Darwin stub, which emerged when trying to build the new Builder targets, so yay.)

PiperOrigin-RevId: 261410344

Author: jonsimantov

admob/src/include/firebase/admob/types.h

@@ -42,7 +42,7 @@ namespace admob {
 #if defined(__ANDROID__)
 /// An Android Activity from Java.
 typedef jobject AdParent;
-#elif defined(TARGET_OS_IPHONE)
+#elif defined(TARGET_OS_IPHONE) && TARGET_OS_IPHONE
 /// A pointer to an iOS UIView.
 typedef id AdParent;
 #else