Enable libavb_host_unittests for presubmit

This uses the new support for deviceless tests in TEST_MAPPING that was
just announced, as well as previous work to get libavb_host_unittests
working properly with atest, to enable the AVB host unittests in
presubmit.

Test: 'cd external/avb && atest', verified libavb_host_unittests was run
Change-Id: Ib3b8ae52566509a8b0d6906f91a9447f18085cd9