Replace SystemNotificationChannels on Tethering

Tethering used SystemNotificationChannels for showing
notification before. But SystemNotificationChannels is used for
system package only, tethering package is not able to use it now.
Thus, replace it with tethering own notification channel.

Bug: 122085773
Test: atest TetheringTests
Change-Id: I854a102795c19351a4137360414926c461cc604c