wg: Add and use dummy AppTheme

Preparation for when the app will start carrying
colors reflecting the WireGuard brand.

Signed-off-by: Harsh Shandilya <me@msfjarvis.dev>

app/src/main/AndroidManifest.xml

@@ -16,7 +16,7 @@
         android:label="@string/app_name"
         android:roundIcon="@mipmap/ic_launcher_round"
         android:supportsRtl="true"
-        android:theme="@android:style/Theme.Material.Light.DarkActionBar"
+        android:theme="@style/AppTheme"
         tools:ignore="UnusedAttribute">
 
         <activity android:name=".activity.MainActivity">

app/src/main/res/values/styles.xml

@@ -1,6 +1,7 @@
 <?xml version="1.0" encoding="utf-8"?>
 <resources xmlns:android="http://schemas.android.com/apk/res/android">
 
+    <style name="AppTheme" parent="Theme.AppCompat.Light.DarkActionBar" />
     <style name="fab_label" parent="android:TextAppearance.DeviceDefault.Inverse">
         <item name="android:background">@drawable/fab_label_background</item>
     </style>