Fix forceActiveFocus() with multiple items having focus: true

Fix forceActiveFocus() with multiple items having focus: true

Ensure focus is cleared from an item if it is parented to an item that
already has a sub focus item.  Checking scopedFocusItem() didn't allow
for the case where an unparented item is the root of the focus tree
but not itself a focus scope, checking the unguarded subFocusItem
member will.

Change-Id: I482779e8077e9f282d22bd0090e669840764e52a
Reviewed-by: Bea Lam <bea.lam@nokia.com>