[WifiSetup] Disable app starting window

Disable the app starting window when start WifiSetupActivity so that
there will not be a blank black screen the first time this activity
is started.

Bug: 17457465
Change-Id: I4fa019a603a088d458fa9bcf126364fbb49b1b1f
diff --git a/AndroidManifest.xml b/AndroidManifest.xml
index 0fb6228..a2d80b9 100644
--- a/AndroidManifest.xml
+++ b/AndroidManifest.xml
@@ -193,7 +193,7 @@
 
         <activity android:name=".wifi.WifiSetupActivity"
                 android:taskAffinity="com.android.wizard"
-                android:theme="@style/SetupWizardWifiTheme"
+                android:theme="@style/SetupWizardDisableAppStartingTheme"
                 android:label="@string/wifi_setup_wizard_title"
                 android:icon="@drawable/empty_icon"
                 android:clearTaskOnLaunch="true">
diff --git a/res/values/themes.xml b/res/values/themes.xml
index ef53618..e9d4297 100644
--- a/res/values/themes.xml
+++ b/res/values/themes.xml
@@ -22,6 +22,12 @@
     <attr name="wifi_signal_color" format="reference" />
     <attr name="wifi_signal" format="reference" />
 
+    <style name="SetupWizardDisableAppStartingTheme">
+        <!-- Theme to disable the app starting window. The actual theme of the activity needs to
+             be then set in code via setTheme or onApplyThemeResource. -->
+        <item name="android:windowBackground">@null</item>
+    </style>
+
     <style name="SetupWizardWifiTheme" parent="android:Theme.Material.NoActionBar">
         <item name="android:alertDialogTheme">@style/Theme.WifiDialog</item>
         <item name="android:colorAccent">@color/setup_wizard_color_accent_dark</item>