Fixes primary stack being visible when minimizing 3rd party launcher

When using 3rd party launcher, the home stack is not merged with
overview then when minimizing, it will not be secondary split screen
stack. Then the primary stack will be presumed invisible even though
it is on the screen. The added check will make sure that the docked
stack is visible when minimizing.

Test: use 3rd party launcher, minimize
Change-Id: I5837ac6339352cae926c970f7453f5564abafb70
Fixes: 77818300