Use NetworkStackNext as preinstalled network stack

NetworkStackNext is the build of the network stack targeted at the
next API, e.g. system_current and not system_29.

While NetworkStack is a mainline module that needs to be built with a
stable SDK (system_29), NetworkStackNext is necessary to test
functionalities that require the in-development SDK (system_current).
Using NetworkStackNext preinstalled on development builds allows for
testing the new APIs and functionalities.

NetworkStackNext is also configured with a mainline module package name
on targets that use mainline modules; but it is not intended to receive
mainline updates.

Once system_current is finalized (system_30 ?), NetworkStack can start
using it, and the system image should switch back to NetworkStack
instead of NetworkStackNext.

Bug: 139269711
Test: Built, phone boots, WiFi working
Change-Id: I885b2c441b99daa3165a4a365738682958478291