Add NetworkTemplate get/set transport type methods

This is needed for counter-part modules to query data usage
with specified transport type.
This is kept hidden and will be exposed in follow-up
CLs.

Flag: EXEMPT controlled in the counter-part CL
Test: atest ConnectivityCoverageTests --test-filter \
'NetworkTemplate*' --update-device
Bug: 419123367
Bug: 420531377

Change-Id: I503b4d7307d008a64a14aa713d9714428e399809
5 files changed