Properly cancel Tether notification.

When we add a second type of tethering we unify the notifications
into a single generic type.  This is done by canceling the first
and replacing it, but this flow was improperly canceling, so
you could end up with orphaned tethering icons.

bug:7283605
Change-Id: I1d136f51592b4326d48578cf67b69122e45d4984