Disambiguate NetworkCallback JavaDoc.

The JavaDoc for many of the requestNetwork and
[un]registerNetworkCallback APIs incorrectly mentions the
PendingIntent version of the APIs instead of the NetworkCallback
version.

Also fix a minor issue in the registerDefaultNetworkCallback
JavaDoc: the default network request is an implementation detail,
so don't mention it. Instead, talk about the "system default
network".

Change-Id: Id94d98261daa2bd768c10e033cb8092729b21c91