Run Robolectric tests on build server

Configure the build server task to run 'test' task, and zip the
result XMLs to host-test-reports for tradefed to pick it up.

Test: out/dist/host-test-reports get generated by TreeHugger
Bug: 37677781
Change-Id: I6162ce0996912b7445ef92c44e48625c4fb4e48f