Use a builder for NetworkSecurityConfig

The builder supports all the standard builder set* methods as well as
setting a parent builder to use when values are not set (recursively).
This allows us to have a level of inheretence in configurations without
complicating the lookup and trust checking logic by doing inheretence
when building the configs.

Change-Id: I054af83451e52761227479eadf9cb9803437505f