Slice of Provider Model: add slice structure

Implement a slice which can show/update carrier networks.
Before and after: go/b173971144screenshot

Bug: 173971144
Test: atest NetworkProviderWorkerTest  (PASS)
atest ProviderModelSliceTest  (PASS)

Change-Id: I3f0dab364c88723ef3185a2ff040b1fbd1b099f4
8 files changed