Provide explicit Network via JobParameters.

On devices with multiple active networks, or rapidly switching
between networks, we need an API to tell jobs explicitly which
network to use.  (For example, the default route could meet all
job criteria, but we could have changed the default network by the
time we spun up the JobService.)

This also paves the way for us choosing to run jobs over
non-default networks.

Test: verified via DownloadManager
Bug: 64133169
Change-Id: Ic8d654707e39236c8da85a5e172161ac39e5f0b3