Bug 882056 - [Dialer] Single call on hold can only be performed from BT devices

* Now the single call on hold can only be triggered from bluetooth devices. If you want to test without bluetooth device, removing https://github.com/mozilla-b2g/gaia/pull/10976/files#L0R652 and https://github.com/mozilla-b2g/gaia/pull/10976/files#L0R654 can bring back tapping.

Screenshots:
![device-2013-07-15-201921](https://f.cloud.github.com/assets/2406290/797456/322fc608-ed49-11e2-89d3-aef03a0e83e2.png)
![device-2013-07-15-201946](https://f.cloud.github.com/assets/2406290/797457/33f58b44-ed49-11e2-8aa6-a8d650958432.png)