Make tether settings intent explicit

Currently tethering notification is sending a pending intent for
redirecting user to tether settings page. However, this intent
is implicit that only create with
Settings.ACTION_TETHER_SETTINGS. For security reasons, this
intetnt should specify an explicit component to be delivered to.
Thus, specify the settings package name to this intent.

Test: atest TetheringTests
Bug: 156353008
Change-Id: I49187aee8a004caa890e2a73c0a28d280215c7d4