feat(android): bump firebase-android-sdk to v26.2.0

one symbol in underlying native code changed, but this is not breaking,
the previous commit to move firebase-ios-sdk contained the only breaking
change that was visible in the Dart code / Flutter API

Author: mikehardy

packages/firebase_core/firebase_core/android/gradle.properties

@@ -1,2 +1,2 @@
-FirebaseSDKVersion=25.13.0
+FirebaseSDKVersion=26.2.0
 android.enableR8=true

packages/firebase_remote_config/android/src/main/java/io/flutter/plugins/firebase/firebaseremoteconfig/MethodCallHandlerImpl.java

@@ -134,7 +134,7 @@ public void onComplete(Task<Boolean> task) {
       case "RemoteConfig#setDefaults":
         {
           Map<String, Object> defaults = call.argument("defaults");
-          FirebaseRemoteConfig.getInstance().setDefaults(defaults);
+          FirebaseRemoteConfig.getInstance().setDefaultsAsync(defaults);
           SharedPreferences.Editor editor = sharedPreferences.edit();
           editor.putStringSet(DEFAULT_PREF_KEY, defaults.keySet()).apply();
           result.success(null);