Fix unlock transition double offset nested tasks

Use parent-based position to prevent nested tasks being offsetted
unexpectedly with screen-based position when performing unlock
transition.

Fix: 278282153
Test: http://recall/-/fLARJNt42LVxc3tt86SneW/b9gvDwL12TaDaLEkH4tUCS
Change-Id: Ib95230742a52e0a9d6b2b1f791a11eec628b179b