This change simply adds a new constant, `TRANSPORT_LOWPAN`, for
identifying low-power wireless networks like Thread.
Bug: b/33073713
Merged-In: Ie4aa77496f8ff466fa1a5fbc556e9c029457a689
(cherry pick from commit 103292d0b7)
Change-Id: I21f9b41b8b31c63ceeb1bc9c965f6da2614c356a
Test: runtest frameworks-net (not in original commit message)