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
Change-Id: I58a6e1f27aff052050197d1901f43a98d7aa1167