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

Original change: https://android-review.googlesource.com/c/platform/frameworks/base/+/1329013

Change-Id: Ia843da82bc8fa03aca33a51fc309b65e73dfdc8f
Merged-In: I4c3af75c87b797bcde9356a94c835c7422dac1c6
(cherry picked from commit d7638aec)