Fix ClassCastException

In "Change I95ed2aae: Stop using shared DUN APN when tethering stops",
sha1: 8beff958, an assumption is made
that the msg.obj is always an ApnContext, this is not true for CDMA.
Eventually we plan on removing the destinction between GSM and CDMA but
for now we need to handle it handle it.

Bug: 5904734
Change-Id: I86873dc7aeda5234c14a6fe1e4ec7345ee30e957