Bug 897015 - B2G Emulator: support switching preferred network type in console command.

Please see [bug 897015](https://bugzilla.mozilla.org/show_bug.cgi?id=897015).

Currently the preferred Network Type is GSM/WCDMA, so it is failed to switch to cdma by using 'modem tech' console command.
To fix this, maybe we could consider to make 'modem tech' can also support switching preferred network type.
So I extent 'modem tech' command as below:
* ```modem tech```: allows you to display the current state of emulator modem.
* ```modem tech <technology>```: allows you to change the technology of emulator modem.
* ```modem tech <technology> <mask>```: allows you to change the technology and preferred mask of emulator modem.