Merge changes from topic "sp04"

* changes:
  [SP05] add unit test for onStatsProviderLimitReached in NPMS
  [SP04] add unit test for NetworkStatsProvider
  [SP03] support registerNetworkStatsProvider API
  [SP03.1] Replace com.android.internal.util.Preconditions.checkNotNull
tree: 9aca0e4dca91372369f32bf928b3b2cce0b9c61a
  1. core/
  2. services/
  3. tests/