PANS - Support unreachable default network

Framework provides several preferences in PANS feature. To meet those
preferences, Netd needs to support two operations for framework:

(1) Set OEM-paid network as default network for apps.
(2) Prohibit apps to use default network if it is not explicitly
selected.

The #1 is supported by previous commit already. This commit implements
the #2, which adds a new IP rule priority for unconnected socket, reuses
existing IP rule priorities in explicit and implicit network selection.
Rules are looks like:

15000:	from all fwmark 0x10034/0x1ffff iif lo uidrange x-y unreachable
...
22000:	from all fwmark 0x34/0x1ffff iif lo uidrange x-y unreachable
...
27000:	from all fwmark 0x0/0xffff iif lo uidrange x-y unreachable

An UNREACHABLE network (netId 52) is created for framework to specify
that the default network is unavailable for designated apps.

Bug: 181579204
Test: atest
Change-Id: I21530928a85870df673e2d1387fde130fe5a0104