Add phony name for robolectric tests

Robolectric tests are installed by Soong, which means Make doesn't
know about the install location to create the phony rules.  Add
a phony rule in Soong for now until Soong's installation rules are
fleshed out.

Bug: 184946827
Test: m SettingsRoboTests
Change-Id: I60d76474949800e1a737714d86df84eff9a92214