This change simply adds a new constant, `TRANSPORT_LOWPAN`, for
identifying low-power wireless networks like Thread.
Bug: b/33073713
Test: builds
Change-Id: I50d9b8df1a0270e06c28f001adf6c52a142a85af
Merged-In: Ie4aa77496f8ff466fa1a5fbc556e9c029457a689
(cherry pick from commit 103292d0b7)