Detect NetworkStack path in rollback test

The network stack module may be implemented by NetworkStack (updatable
version built against stable SDK) or NetworkStackNext (test version
shipped only in system images, built against in-development SDK).

Bug: 144168912
Test: atest StagedRollbackTest
Merged-In: I1788e323f763ca88b69b082f38078bee3bfe480d
Change-Id: I963cc20ae4356f1728f6dd1bbf14f48ded978756