Wait for keyguard showing rather than app not visible when going to sleep.

This was (only sometimes) causing the sleep transition to be cancelled, which then caused test flakes since we're asserting an ordering of app visibility states, not just a correct end state.

Bug: 278086361
Test: atest SystemUITests / this test
Flag: N/A
Change-Id: I3693feca80195a037fbbefd3bfff81988825a98f