Add SetDefaultEventParameters and ClearDefaultEventParameters to Analytics

This change introduces two new C++ Analytics SDK functions:
- `SetDefaultEventParameters(const std::map<std::string, firebase::Variant>& params)`: Allows setting default parameters that will be included in all subsequent `LogEvent` calls. If a `firebase::Variant::Null()` is provided as a value for a key, that specific default parameter will be cleared/removed.
- `ClearDefaultEventParameters()`: Clears all currently set default event parameters.

Platform implementations:
- iOS: Uses `[FIRAnalytics setDefaultEventParameters:]`. `firebase::Variant::Null()` maps to `[NSNull null]` for clearing individual parameters. Calling with `nil` clears all.
- Android: Uses `FirebaseAnalytics.setDefaultEventParameters(Bundle)`. `firebase::Variant::Null()` results in `Bundle.putString(key, null)`. Calling with a `null` Bundle clears all.
- Stub: Implemented as no-ops.

Unit tests and integration tests have been updated to cover these new functionalities, including the null-value handling for clearing specific parameters and the overall clearing of all parameters. Integer literals in integration tests use standard int types.

Author: google-labs-jules[bot]

analytics/integration_test/src/integration_test.cc

@@ -347,7 +347,7 @@ TEST_F(FirebaseAnalyticsTest, TestDefaultEventParametersUsage) {
   std::map<std::string, firebase::Variant> initial_defaults;
   initial_defaults["initial_key"] = "initial_value";
   initial_defaults["key_to_be_nulled"] = "text_before_null";
-  initial_defaults["numeric_default"] = 12345LL;
+  initial_defaults["numeric_default"] = 12345;
 
   LogInfo("Setting initial default event parameters.");
   firebase::analytics::SetDefaultEventParameters(initial_defaults);
@@ -379,7 +379,7 @@ TEST_F(FirebaseAnalyticsTest, TestClearDefaultEventParametersFunctionality) {
 
   std::map<std::string, firebase::Variant> defaults_to_clear;
   defaults_to_clear["default_one"] = "will_be_cleared";
-  defaults_to_clear["default_two"] = 9876LL;
+  defaults_to_clear["default_two"] = 9876;
 
   LogInfo("Setting default parameters before clearing.");
   firebase::analytics::SetDefaultEventParameters(defaults_to_clear);