Generate NetworkStackNext manifest from xml

Instead of a genrule, use an actual manifest merge with a separate
AndroidManifest_Next.xml for NetworkStackNext.

This is necessary as the tooling does not support injecting the train
version number to a genrule, it needs an actual manifest file.

Bug: 148907674
Test: m NetworkStackNext, diff aapt dump xmltree from parent build
Change-Id: I84d97e9108616cac1bd737dc0e4f250cc96a73da