Switch the NetworkInfo.mIsAvailable default

The placeholder for disconnected networks was setting it to false, but
this technically means that we know an attempt to connect to that
network will fail (which we don't really now).  Some applications use
this an decide not to bother trying - an MMS app for example would
never send an MMS because it thinks the network is never available.

This is a L regression.

bug:17669247
Change-Id: Id6041f226da069c267c56418d42c55978c36c66f