Modify GMA minimum Android SDK to 21.

Author: jonsimantov

gma/integration_test/AndroidManifest.xml

@@ -22,7 +22,7 @@
   <uses-permission android:name="android.permission.INTERNET" />
   <uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />
   <uses-permission android:name="android.permission.WAKE_LOCK" />
-  <uses-sdk android:minSdkVersion="16" android:targetSdkVersion="28" />
+  <uses-sdk android:minSdkVersion="21" android:targetSdkVersion="28" />
   <application android:label="@string/app_name">
     <!-- You may replace the sample App ID below with your own App ID. -->
     <meta-data

gma/integration_test/build.gradle

@@ -54,7 +54,7 @@ android {
 
   defaultConfig {
     applicationId 'com.google.android.admob.testapp'
-    minSdkVersion 19
+    minSdkVersion 21
     targetSdkVersion 28
     versionCode 1
     versionName '1.0'