Refactor setup wizard-related code

Extracted setup-related code from WifiSettings, putting it into a new
derived class, WifiSettingsForSetupWizard.

Set taskAffinity to com.android.wizard, for alignment with other
setup wizard-related tasks.

Change-Id: If371f219dac5d3eff8f7899c16cc6f9475be47cf
diff --git a/AndroidManifest.xml b/AndroidManifest.xml
index 4388f0b..ee78672 100644
--- a/AndroidManifest.xml
+++ b/AndroidManifest.xml
@@ -169,6 +169,7 @@
         </activity>
 
         <activity android:name=".wifi.WifiSetupActivity"
+                android:taskAffinity="com.android.wizard"
                 android:theme="@style/SetupWizardWifiTheme"
                 android:label="@string/wifi_setup_wizard_title"
                 android:icon="@drawable/empty_icon"