Hold wifi and mobile interfaces since boot in NetworkStatsService

In current design, the interface will be removed from the list if
the network disconnected which will cause getUidStatsForTransport()
returns unexpected snapshot to caller since the list is empty.

This change also remove interface name from all entries before
the method returns the result.

Ignore-AOSP-First: non-AOSP CL is included in the same topic
Bug: 231514741
Test: FrameworksNetTests
      manual test
Change-Id: Ie60829a65d0d9b5b63ad353695a820c0586e3665