Bump minSdkVersion to 21 for tests app

Author: nilsreichardt

tests/android/app/build.gradle

@@ -47,7 +47,7 @@ android {
     defaultConfig {
         // TODO: Specify your own unique Application ID (https://developer.android.com/studio/build/application-id.html).
         applicationId "io.flutter.plugins.firebase.tests"
-        minSdkVersion 19
+        minSdkVersion 21
         targetSdkVersion flutter.targetSdkVersion
         versionCode flutterVersionCode.toInteger()
         versionName flutterVersionName