Iterate over a copy in QSTileHost#changeTiles

Prevent OOB exception when iterating over the list of previous tiles, as
it could be modified.

As we just need to find out what changed, there's no need to add extra
locking.

Test: no test
Fixes: 150212857
Change-Id: I129660a81771acba3d6a5da58fb14ed19659d4ea