Minor cleanups to NetworkStats interface tracking.

Currently, when a network goes into CONNECTED state, we call
updateLinkProperties and then notifyIfacesChangedForNetworkStats.
The latter is unnecessary, as there are exactly two cases:

1. networkAgent.linkProperties != null: updateLinkProperties will
   call notifyIfacesChangedForNetworkStats, because oldLp is null
   and networkAgent.linkProperties is not null.
2. networkAgent.linkProperties is null: there is no need to call
   notifyIfacesChangedForNetworkStats, because no interfaces were
   added or removed. When they are, updateLinkProperties will be
   called again.

Removing the call to notifyIfacesChangedForNetworkStats avoids
a stats poll, which is a minor performance improvement.

Also, remove the NetworkStatsService code to do asynchronous
interface updates, since it has no callers.

Bug: 72107146
Test: builds, boots
Test: runtest frameworks-net
Change-Id: I9337ea26c0505a1c66ceda01254b68e25cd7972c