Make requests for restricted networks not require unrestricted access.

Currently, calling startUsingNetworkFeature for a restricted APN
type (e.g., IMS or FOTA) will create a request that requires
NET_CAPABILITY_NOT_RESTRICTED. Because these APNs are restricted,
when we bring them up we conclude that it does not match the
unrestricted requirement, and we tear them down.

1. Clear the NET_CAPABILITY_NOT_RESTRICTED capability when
   creating requests in startUsingNetworkFeature.
2. Refactor the code to a common function so this cannot happen
   again.

Bug: 15191336
Change-Id: Id1ec79c58ff79b1a83457ffaecc57d50b61ed4e4