SystemUI: Enhancement for DDS switch on quick setting panel

1. Start an AsyncTask to set default data subscription to avoid ANR while
switching DDS in status bar continuously.
2. Make the DDS switch button grayed out while changing subscription.
3. Hide APN switch button if DDS is set to non default subscription.

CRs-Fixed: 677798

Change-Id: I36de2fa45921fb123ab24d94696fba3fe4d48656