Remove NETWORK_STACK_SERVICE constant

Instead, have a dedicated method in android.net.NetworkStack allowing to
fetch the stable AIDL token for the service.

This avoids returning IBinder from getSystemService, as getSystemService
should generally return manager classes.

Test: atest FrameworksNetTests NetworkStackTests
Fixes: 151243982
Merged-In: I58a6e1f27aff052050197d1901f43a98d7aa1167
(clean cherry-pick from internal branch)

Change-Id: I75aba269595f3e315dd5e0693c878b2026e8e299