Ignore invisible tasks in TaskLaunchParamsModifier conflict avoidance

TaskLaunchParamsModifier avoids placing freeform tasks on top of
each other via the `adjustBoundsToAvoidConflictInDisplayArea` method by
detecting the existing freeform tasks in the display area. However, this
does not take into account a task visibility just its window mode.
Instead, invisible tasks should be ignored to prevent a tasks bounds
from being adjust for no reason as there is not visible window to
conflict.

Flag: EXEMPT bug fix
Test: atest WmTests:TaskLaunchParamsModifierTests
Fixes: 425921989
Change-Id: Ib4a28ddb505882b3dd3b2fa24761eb868345e065