Exclude okhttp from flipper dependency (#29260)

Summary:
This fixes #28481. As explained in [this comment](#28481 (comment)), the flipper network plugin pulls a more recent version of okhttp (3.14), but only versions of okhttp up to 3.12 works on Android API 21 and less.

This prevented being able to run the app in debug mode, it was still working fine in release mode.

## Changelog

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[Android] [Fixed] - Fix unable to run in debug mode on Android API < 21

Pull Request resolved: #29260

Test Plan:
Using `yarn react-native run-android` the app would instantly crash with this error in `adb logcat`:
```
E/AndroidRuntime( 5079): java.lang.RuntimeException: Unable to create application com.awesometsproject.MainApplication: java.lang.RuntimeException: Requested enabled DevSupportManager, but DevSupportManagerImpl class was not found or could not be created
E/AndroidRuntime( 5079):        at android.app.ActivityThread.handleBindApplication(ActivityThread.java:4154)
E/AndroidRuntime( 5079):        at android.app.ActivityThread.access$1300(ActivityThread.java:130)
E/AndroidRuntime( 5079):        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1255)
E/AndroidRuntime( 5079):        at android.os.Handler.dispatchMessage(Handler.java:99)
E/AndroidRuntime( 5079):        at android.os.Looper.loop(Looper.java:137)
E/AndroidRuntime( 5079):        at android.app.ActivityThread.main(ActivityThread.java:4745)
E/AndroidRuntime( 5079):        at java.lang.reflect.Method.invokeNative(Native Method)
E/AndroidRuntime( 5079):        at java.lang.reflect.Method.invoke(Method.java:511)
E/AndroidRuntime( 5079):        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:786)
E/AndroidRuntime( 5079):        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
E/AndroidRuntime( 5079):        at dalvik.system.NativeStart.main(Native Method)
E/AndroidRuntime( 5079): Caused by: java.lang.RuntimeException: Requested enabled DevSupportManager, but DevSupportManagerImpl class was not found or could not be created
E/AndroidRuntime( 5079):        at com.facebook.react.devsupport.DevSupportManagerFactory.create(DevSupportManagerFactory.java:90)
E/AndroidRuntime( 5079):        at com.facebook.react.ReactInstanceManager.<init>(ReactInstanceManager.java:238)
E/AndroidRuntime( 5079):        at com.facebook.react.ReactInstanceManagerBuilder.build(ReactInstanceManagerBuilder.java:281)
E/AndroidRuntime( 5079):        at com.facebook.react.ReactNativeHost.createReactInstanceManager(ReactNativeHost.java:87)
E/AndroidRuntime( 5079):        at com.facebook.react.ReactNativeHost.getReactInstanceManager(ReactNativeHost.java:39)
E/AndroidRuntime( 5079):        at com.awesometsproject.MainApplication.onCreate(MainApplication.java:47)
E/AndroidRuntime( 5079):        at android.app.Instrumentation.callApplicationOnCreate(Instrumentation.java:999)
E/AndroidRuntime( 5079):        at android.app.ActivityThread.handleBindApplication(ActivityThread.java:4151)
E/AndroidRuntime( 5079):        ... 10 more
E/AndroidRuntime( 5079): Caused by: java.lang.reflect.InvocationTargetException
E/AndroidRuntime( 5079):        at java.lang.reflect.Constructor.constructNative(Native Method)
E/AndroidRuntime( 5079):        at java.lang.reflect.Constructor.newInstance(Constructor.java:417)
E/AndroidRuntime( 5079):        at com.facebook.react.devsupport.DevSupportManagerFactory.create(DevSupportManagerFactory.java:80)
E/AndroidRuntime( 5079):        ... 17 more
E/AndroidRuntime( 5079): Caused by: java.lang.NoClassDefFoundError: java.util.Objects
E/AndroidRuntime( 5079):        at okhttp3.CertificatePinner.withCertificateChainCleaner(CertificatePinner.java:231)
E/AndroidRuntime( 5079):        at okhttp3.OkHttpClient.<init>(OkHttpClient.java:238)
E/AndroidRuntime( 5079):        at okhttp3.OkHttpClient$Builder.build(OkHttpClient.java:1015)
E/AndroidRuntime( 5079):        at com.facebook.react.devsupport.DevServerHelper.<init>(DevServerHelper.java:132)
E/AndroidRuntime( 5079):        at com.facebook.react.devsupport.DevSupportManagerImpl.<init>(DevSupportManagerImpl.java:183)
E/AndroidRuntime( 5079):        ... 20 more
W/ActivityManager( 1456):   Force finishing activity com.awesometsproject/.MainActivity
```

With this fix, the app launch successfully in debug mode, without having to remove flipper altogether from our config.

Reviewed By: passy

Differential Revision: D22521109

Pulled By: mdvacca

fbshipit-source-id: 3c0263642438bd7c0d09b045e15a933bd8a26734

Author: Shywim

template/android/app/build.gradle

@@ -193,6 +193,7 @@ dependencies {
 
     debugImplementation("com.facebook.flipper:flipper-network-plugin:${FLIPPER_VERSION}") {
         exclude group:'com.facebook.flipper'
+        exclude group:'com.squareup.okhttp3', module:'okhttp'
     }
 
     debugImplementation("com.facebook.flipper:flipper-fresco-plugin:${FLIPPER_VERSION}") {