Allow NetworkFactories to match any network specifier on a request

Current usage of NetworkSpecifier: network factory will match a request
if the request has either a (1) empty network specifier, or (2) a
network specifier which is identical to that of the network factory.
Note: 'matching' w.r.t. network specifier - all other matching rules
are still in effect.

Change: add rule (3) or the network specifier of the network factory
is the special string (which is defined as "*" and which user-facing
network requests aren't allowed to use).

Rationale: allows on-demand network creation.

Example:
- Can specify a Wi-Fi NetworkRequest with NetworkSpecifier="ssid"
- It will match a Wi-Fi network factory specifying NetworkSpecifier="*"
- That network factory will bring up a Wi-Fi network, connecting to the
  specified SSID.
- Once the network is created it will create a NetworkAgent which will
  now have a NetworkSpecifier matching that of the request (not the
  match-all special string!)

That final step of making sure that the NetworkAgent matches the request
and not the match-all is critical to delivering any subsequent callbacks
correctly. I.e. your network will only get callbacks which match it.

Bug: 26192833
Change-Id: I49e3b492e0bb48a3f6e9a34e3f94f0e1cf89741f