Sample app requires API 21+

Summary:
Currently running the sample app on a pre-21 device produces a runtime error. This is because it includes OkHttp which requires 21+.

So this change just stops the sample app from being installable on pre-21 devices.

```
FATAL EXCEPTION: main
Process: com.facebook.flipper.sample, PID: 19629
java.lang.ExceptionInInitializerError
	at okhttp3.OkHttpClient.newSslSocketFactory(OkHttpClient.java:263)
	at okhttp3.OkHttpClient.<init>(OkHttpClient.java:229)
	at okhttp3.OkHttpClient$Builder.build(OkHttpClient.java:1015)
	at com.facebook.flipper.sample.FlipperSampleApplication.onCreate(FlipperSampleApplication.java:54)
	at android.app.Instrumentation.callApplicationOnCreate(Instrumentation.java:1007)
	at android.app.ActivityThread.handleBindApplication(ActivityThread.java:4344)
	at android.app.ActivityThread.access$1500(ActivityThread.java:135)
	at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1256)
	at android.os.Handler.dispatchMessage(Handler.java:102)
	at android.os.Looper.loop(Looper.java:136)
	at android.app.ActivityThread.main(ActivityThread.java:5017)
	at java.lang.reflect.Method.invokeNative(Native Method)
	at java.lang.reflect.Method.invoke(Method.java:515)
	at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
	at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
	at dalvik.system.NativeStart.main(Native Method)
Caused by: java.lang.IllegalStateException: Expected Android API level 21+ but was 19
	at okhttp3.internal.platform.AndroidPlatform.buildIfSupported(AndroidPlatform.java:238)
	at okhttp3.internal.platform.Platform.findPlatform(Platform.java:202)
	at okhttp3.internal.platform.Platform.<clinit>(Platform.java:79)
	... 16 more
```

Reviewed By: passy

Differential Revision: D14953859

fbshipit-source-id: 8289c19e837f1d3739e120dc5a0f53111b800ca0

android/sample/build.gradle

@@ -4,7 +4,7 @@ android {
     compileSdkVersion rootProject.compileSdkVersion
     buildToolsVersion rootProject.buildToolsVersion
     defaultConfig {
-        minSdkVersion rootProject.minSdkVersion
+        minSdkVersion 21
         testInstrumentationRunner 'androidx.test.runner.AndroidJUnitRunner'
         applicationId 'com.facebook.flipper.sample'
         targetSdkVersion 28