Use NetworkStack.getService in NetworkStackClient

This avoids returning IBinder in Context.getSystemService, having a
dedicated getter for the NetworkStack AIDL interface instead.

Test: atest FrameworksNetTests NetworkStackTests
Bug: 151243982
Merged-In: Ibeaeff5890631e14581a084863d9aeb3178c5d5c
(clean cherry-pick from internal branch)

Change-Id: I7671358ef8f9dc14ad498c4958d027d8c3dba574