Migrate from interop pods to source_files header access (#5760)

FirebaseInstanceID.podspec

@@ -28,11 +28,13 @@ services.
   s.prefix_header_file = false
 
   base_dir = "Firebase/InstanceID/"
-  s.source_files = base_dir + '**/*.[mh]'
+  s.source_files = [
+    base_dir + '**/*.[mh]',
+    'FirebaseCore/Sources/Private/*.h',
+  ]
   s.requires_arc = base_dir + '*.m'
   s.public_header_files = base_dir + 'Public/*.h', base_dir + 'Private/*.h'
   s.private_header_files = base_dir + 'Private/*.h'
-  s.preserve_paths = 'FirebaseCore/Sources/Private/*.h'
   s.pod_target_xcconfig = {
     'GCC_C_LANGUAGE_STANDARD' => 'c99',
     'GCC_PREPROCESSOR_DEFINITIONS' => 'FIRInstanceID_LIB_VERSION=' + String(s.version),