wifi: Set subscriberId for the merged wifi network

The merged wifi network needs subscriberId to link the correct usage
data. Fill subscriberId in NetworkAgentConfig

Note: It needs to merge after aosp/1556552

Bug: 176396812
Test: atest -c FrameworksWifiTests
Test: Manual test to verify NetworkIdentity can get subscriberId.
Change-Id: Id1bf60b010daaf06c9d1bee04661213b2b4bf51e
3 files changed