Update bootstat_test to really work host-side

bootstat_test 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 bootstat_tests --host
Bug: 179092189
Change-Id: Ib6694f760d9ef5f867cd1982f7e1fe65ce9dd62d