[vts-core] Add libdm_test in vts-core

Add libdm_test to vts-core suite and skip the test if the
first-api-level < 29.

Bug: 132702215
Test: atest vts_libdm_test
Change-Id: Ic805a4c08e5bf9f6c69019d4ea01825ae888e1fe
2 files changed