Pass around struct android_net_context for better behaviour

Pass around struct android_net_context for better behaviour

Group network context elements in to a single struct and
add a version of android_getaddrinfofornet() that accepts it.

The introduction of UID-based routing means that the UID is an
integral part of the network context when evaluating connectivity,
sorting addresses, etc.

Also, introduce a distinction between DNS netids/marks and those
expected to be used by the application.  This can be important
when the network an application is using is not the same as the
network on which DNS queries will be issued.

Additionally, de-duplicate the UDP connect logic (collapse both
_test_connect() and _find_src_addr() into just the latter).

Bug: 19470192
Bug: 20733156
Bug: 21832279
Change-Id: If16c2f4744695f507993afdac078ca105eb5d3e4