Correct PendingIntent#getActivity() flags

TetheringNotificationUpdater create a PendingIntent with
Intent#FLAG_ACTIVITY_NEW_TASK flag. But this flag is used for
Intent only. Thus, move this flag into Intent.

Bug: 158639789
Test: atest TetheringTests
Change-Id: I4c3af75c87b797bcde9356a94c835c7422dac1c6