flipper: Enable Flipper.

Call the appropriate initialize-Flipper function in native runtime
code.

Part of the RN v0.61 -> v0.62 changes to the template app [1],
corresponding to facebook/react-native@05f5cb534. This should be
fine to do after the main upgrade commit; nothing in React Native's
internals should depend on Flipper being enabled [2].

On iOS, use the form of the initialize-Flipper function as it was
updated in facebook/react-native@b4d1fcfb2 and wrap the call site in
a conditional as done there.

[1] https://react-native-community.github.io/upgrade-helper/?from=0.61.5&to=0.62.2
[2] #4244 (comment)

android/app/src/main/java/com/zulipmobile/MainApplication.java

@@ -78,6 +78,7 @@ public void onCreate() {
         super.onCreate();
         FCMPushNotifications.createNotificationChannel(this);
         SoLoader.init(this, /* native exopackage */ false);
+        initializeFlipper(this, getReactNativeHost().getReactInstanceManager());
         conversations = new ConversationMap();
     }
 

ios/ZulipMobile/AppDelegate.m

@@ -36,6 +36,10 @@ @implementation AppDelegate
 
 - (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
 {
+  #if DEBUG
+    InitializeFlipper(application);
+  #endif
+
   RCTSetLogThreshold(RCTLogLevelError);
   self.moduleRegistryAdapter = [[UMModuleRegistryAdapter alloc] initWithModuleRegistryProvider:[[UMModuleRegistryProvider alloc] init]];