Add Trust to NetworkCapabilities.

Indicates the user has indicated implicit trust of a network.  This
generally means it's a sim-selected carrier, a plugged in ethernet,
a paired BT device or a wifi they've asked to connect to.  Untrusted
networks are probably limited to unknown wifi AP.

Change-Id: I89490bdaa3c2d63d33f876c72d8b088dc155fa3d