[Thread] Fix the overlap of top app bar and the system's status bar in demo app

Before: https://screenshot.googleplex.com/7NJy8izdNKJoa4u
After: https://screenshot.googleplex.com/V4PyKzkQbwBVJw8

Change-Id: I3073541ea02ae9936fef728f04aece1d6389c5e8

thread/demoapp/res/layout/main_activity.xml

@@ -21,6 +21,7 @@
     android:id="@+id/drawer_layout"
     android:layout_width="match_parent"
     android:layout_height="match_parent"
+    android:fitsSystemWindows="true"
     tools:context=".MainActivity">
 
     <LinearLayout

thread/demoapp/res/layout/thread_network_settings_fragment.xml

@@ -17,8 +17,7 @@
 <ScrollView
   xmlns:android="http://schemas.android.com/apk/res/android"
   android:layout_width="match_parent"
-  android:layout_height="match_parent"
-  android:fitsSystemWindows="true">
+  android:layout_height="match_parent">
     <LinearLayout
         xmlns:app="http://schemas.android.com/apk/res-auto"
         xmlns:tools="http://schemas.android.com/tools"