[VCN07.6] Add NOT_VCN_MANAGED for NetworkAgent implementations

Follow-up from aosp/1549897, the capability will not be in
default capabilities, but will be in most of the NetworkRequests
which requires internet. Thus, add the capabilities into all
NetworkAgent implementations by default to provide
backward compatibility.

Test: TH
Bug: 175662146
Change-Id: Id706fea7ac3136779e51853956bc1251f1f54611