Disable recents onboarding for test automation

The recents onboarding blocks UI automated tests. It should be disabled
if we're running in a test harness.

Bug: 73549472

Test: Tried permutations of setting persist.quickstep.onboarding.enabled
and ro.test_harness to true/false and verified the recents onboarding
pop up was present/absent when expected.

Change-Id: I7502c1873619db19294658b2b7a4ddc64f8d3b4e