Fix example to build on Android

Qt 5.13 raised the min SDK version for Qt for Android to v21.

Change-Id: Iba9d0446f7d74301252a953caedbc83b1ecc983d
Reviewed-by: BogDan Vatra <bogdan@kdab.com>

examples/androidextras/notification/android-sources/AndroidManifest.xml

@@ -36,7 +36,7 @@
             <!-- Splash screen -->
         </activity>
     </application>
-    <uses-sdk android:minSdkVersion="16" />
+    <uses-sdk android:minSdkVersion="21" />
     <supports-screens android:largeScreens="true" android:normalScreens="true" android:anyDensity="true" android:smallScreens="true"/>
     <!-- %%INSERT_PERMISSIONS -->
     <uses-permission android:name="android.permission.INTERNET"/>