Use NetworkStack instead of NetworkStackNext

Now that NetworkStack has bumped its target SDK to 31, use it
instead of the development SDK NetworkStackNext binary.

In practice NetworkStack and NetworkStackNext are currently
identical, as "current" is now SDK 31 in S branches, and
NetworkStackNext does not contain any T-specific code yet.

This change should not be merged in development branches downstream of
AOSP.

Bug: 189581241
Test: device boots, has connectivity
Merged-In: Ia9f58c5986c717cb2882e2fc4daadb2b3874c6b5
Change-Id: I990c76da41d56c8756835b5743e1666d1382f796