Replace StringNetworkSpecifier & subId with TelephonyNetworkSpecifer

StringNetworkSpecifier is widely used to specify subscription id for
the NetworkRequest. The raw string field introduce ambiguity and leave
large space for bugs. With TelephonyNetworkSpecifer, we are able to
settle down the type and value of the fields (although currently only
one) and introduce validation to detect the bug in the beginning.

Bug: 145993724
Test: atest FrameworksNetTests FrameworksTelephonyTests &
      make offline-sdk-docs
Change-Id: Iefbad9b1deb3de2c0b262d9ce5ae0704a50d08a6