[wm]: Changed an order to set a target window in TaskPositioner.

The system crash happens in TaskPositioner.unregister()
with the line mClientCallback.unlinkToDeath(this, 0 /* flags */).
Because mClientCallback has not been initialized before starting window drag.

Send the target window at the register function not at the start function.

Test: manual - Drag freefrom windowing task repeatedly
Test: atest WmTests:TaskPositionerTests
Bug: 142746031
Change-Id: Ied605f35d8b50540ad01056fa87bb2637135f1ab
(cherry picked from commit ac5ad99b44ea26aae1571de36c039eed81e886cf)