existing mobile broadband connections not correctly captured by network settings

[Kristin Chuang]

[Steps]
1. Go to System settings > Network > Mobile Broadband
2. Check the options available in "Network" pull-down menu
3. Click on the pull-down menu and click on the "Add new connection" that shows up
4. Create a new mobile broadband connection
5. Check what options are available in "Network" pull-down menu now
6. Exit System settings
7. Repeat step 1~5

[Expected]
* Existing mobile broadband connections should be listed in the "Network" pull-down menu in step 2.

[Actual]
[x] "Network" pull-down menu in step 2 shows a blank menu as default. Clicking on the menu activates the "Add new connection" wizard, which guides you through creation of a new connection.

[o] Once this new connection is created (step 5), the contents of the menu return to its expected state: this new connection, along with all existing connections will show up in the pull-down menu, and a "add new connection" option at the bottom of the list.

[x] However, once the user exits the system setup and returns (2nd+ attempt of step 2), the pull-down list becomes blank again.

[System]
Xenial, kernel 4.4.0-98-generic
unity-settings-daemon 15.04.1+16.04.20160209-0ubuntu1