fix android manifest for sample

Author: sbSteveK

samples/Android/app/src/main/AndroidManifest.xml

@@ -4,7 +4,8 @@
   <application
       android:allowBackup="true"
       android:label="@string/app_name"
-      android:supportsRtl="true">
+      android:supportsRtl="true"
+      android:theme="@style/AppTheme">
 
     <activity
         android:name="software.amazon.awssdk.iotsamples.MainActivity"