Make firebase::kDefaultAppName const.

The Google C++ Style guide says that names starting with a leading "k"
should be for constants, i.e. for variables declared "constexpr" or "const"
and whose value is fixed for the duration of the program.  But before
this change, kDefaultAppName was not declared "const", and could be modified.

This change is not strictly backwards compatible; any programs which did attempt
to modify kDefaultAppName would break.  However, kDefaultAppName is not documented
in the API documentation, so I think this is OK.  Also a Google search for
"kDefaultAppName" and "firebase" didn't get any hits other than the Firebase API docs
and the Firebase source code, so don't think there is any open-source code which
is doing this.

PiperOrigin-RevId: 256182269

Author: Googler

app/src/app_android.cc

@@ -137,7 +137,7 @@ METHOD_LOOKUP_DEFINITION(
     "com/google/firebase/platforminfo/GlobalLibraryVersionRegistrar",
     GLOBAL_LIBRARY_VERSION_REGISTAR_METHODS)
 
-const char* kDefaultAppName = "__FIRAPP_DEFAULT";
+const char* const kDefaultAppName = "__FIRAPP_DEFAULT";
 
 namespace {
 

app/src/app_desktop.cc

@@ -45,7 +45,7 @@ static std::string g_default_config_path;  // NOLINT
 }  // namespace internal
 
 // The default App name, the same string as what are used for Android and iOS.
-const char* kDefaultAppName = "__FIRAPP_DEFAULT";
+const char* const kDefaultAppName = "__FIRAPP_DEFAULT";
 
 App::App() {
   LogDebug("Creating firebase::App for %s", kFirebaseVersionString);

app/src/app_ios.mm

@@ -51,7 +51,7 @@ - (void)setDataCollectionDefaultEnabled:(BOOL)dataCollectionDefaultEnabled;
 
 DEFINE_FIREBASE_VERSION_STRING(Firebase);
 
-const char* kDefaultAppName = "__FIRAPP_DEFAULT";
+const char* const kDefaultAppName = "__FIRAPP_DEFAULT";
 
 App::App() : data_(nullptr) {
   LogDebug("Creating firebase::App for %s", kFirebaseVersionString);

app/src/app_stub.cc

@@ -29,7 +29,7 @@ namespace firebase {
 
 DEFINE_FIREBASE_VERSION_STRING(Firebase);
 
-const char* kDefaultAppName = "default";
+const char* const kDefaultAppName = "default";
 
 App::App() : data_(new internal::FunctionRegistry) {
   LogDebug("Creating firebase::App for %s", kFirebaseVersionString);

app/src/include/firebase/app.h

@@ -102,7 +102,7 @@ enum InitResult {
 };
 
 /// @brief Default name for firebase::App() objects.
-extern const char* kDefaultAppName;
+extern const char* const kDefaultAppName;
 
 /// @brief Options that control the creation of a Firebase App.
 /// @if cpp_examples