Update string for discarding changes to a particular APN's settings.

Previously was unclear whether the changes, or the entire APN was
being discarded. The word "Cancel" more closely implies that the most
recent changes to the screen will be discarded rather than the entire
APN.

This only changes the resource for english. However, many other
languages have a menu_cancel string that seems much closer
to "Cancel" than "Discard" as well.

i.e. http://cs/android/packages/apps/Settings/res/values-zu/strings.xml?l=1029&rcl=a7397bec91183a7a32561fd8d67b0478af6c65bb
http://cs/android/packages/apps/Settings/res/values-es/strings.xml?l=1029&rcl=a7397bec91183a7a32561fd8d67b0478af6c65bb

Bug: 171538880
Test: N/A
Change-Id: I80fde04dbc36078d5c53e68231fcc4e5c3cf94e6