Bug 870237 - [Gaia][Contacts] use navigator.mozIccManager to access SIM Contact functionality.

Please see [bug 870237](https://bugzilla.mozilla.org/show_bug.cgi?id=870237).

I add a checking for IccManager in this patch.
In this way we can make sure 'import SIM contacts' function won't break no matter [bug 859220](https://bugzilla.mozilla.org/show_bug.cgi?id=859220) is landed or not. Otherwise, 'import SIM contacts' function may keep failed until both bugs are landed. ([bug 870237](https://bugzilla.mozilla.org/show_bug.cgi?id=870237) and [bug 859220](https://bugzilla.mozilla.org/show_bug.cgi?id=859220))

Thanks