Add a Builder to NetworkAgentConfig, and make it SystemApi.

Currently, only support the three elements in the config that are
known to be used.

Bug: 138306002
Test: builds, boots
Test: atest FrameworksTelephonyTests
Change-Id: I1d231ec2ddcff97c039bcbc815a39c1d3e26c410
Merged-In: I1d231ec2ddcff97c039bcbc815a39c1d3e26c410