wifi data usage: support to get carrier merged wifi network.

Carrier merged wifi network is a specific cerrier wifi network
which provides the same user experience as mobile.

To support data usage accounting for carrier merged wifi,
the change provide several APIs in NetworkTemplate:

  1. extend buildTemplateWifi so it could be used for matching
     wifi networks with subscriber Id (IMSI).
  2. add buildTemplateCarrier to let
     NetworkPolicyManagerService creates a single policy for
     a given carrier regardless of network type.

Bug: 176396812
Test: atest -c NetworkTemplateTest
Test: atest -c NetworkStatsServiceTest
Test: Manual Test with test code on mobile and wifi network.
      1. buildTemplateCarrier includes the carrier wifi and
         mobile usage
      2. buildTemplateWifi can get carrier wifi usage and support the
         filter via subscriberId

Change-Id: I667b4adf3eec0bdd3a7385109dd8c1fae8e7be32