Update network select page

1. use getPrefContext() to make sure it has correct style
2. Move operation back to bg thread so it won't block UI

Bug: 114749736
Test: RunSettingsRoboTests
Change-Id: I7737ea8b21d44790617b9c4f13a8c4c41037fca4

tests/robotests/src/com/android/settings/network/telephony/NetworkSelectSettingsTest.java

@@ -81,6 +81,9 @@ public class NetworkSelectSettingsTest {
 
         mNetworkSelectSettings = spy(new NetworkSelectSettings());
         doReturn(mContext).when(mNetworkSelectSettings).getContext();
+        doReturn(mPreferenceManager).when(mNetworkSelectSettings).getPreferenceManager();
+        doReturn(mContext).when(mPreferenceManager).getContext();
+
         mNetworkSelectSettings.mTelephonyManager = mTelephonyManager;
         mNetworkSelectSettings.mConnectedPreferenceCategory = mConnectedPreferenceCategory;
         mNetworkSelectSettings.mPreferenceCategory = mPreferenceCategory;