Update idmap2_tests to really work host-side

idmap2_tests is marked host_supported:true but use
a AndroidTest.xml config that can only work for device.

Deleting the explict config to rely on the auto-gen ones.
Adding unit_tests:true to run in presubmit

Test: presubmit, atest idmap2_tests --host
Bug: 179092189
Change-Id: I0b325abc6ce7fc6c777d070bb0fc0e9e8a7cdedd