Ban socket connections between core and vendor

On PRODUCT_FULL_TREBLE devices, non-vendor domains (coredomain) and
vendor domain are not permitted to connect to each other's sockets.
There are two main exceptions: (1) apps are permitted to talk to other
apps over Unix domain sockets (this is public API in Android
framework), and (2) domains with network access (netdomain) are
permitted to connect to netd.

This commit thus:
* adds neverallow rules restricting socket connection establishment,
* temporarily exempts the domains which are currently violating this
  rule from this restriction. These domains are grouped using the new
  "socket_between_core_and_vendor_violators" attribute. The attribute
  is needed because the types corresponding to violators are not
  exposed to the public policy where the neverallow rules are.

Test: mmm system/sepolicy
Bug: 36613996
Change-Id: I458f5a09a964b06ad2bddb52538ec3a15758b003