Fix MultiDisplayVeiledResizeTaskPositionerTest failure

Kotlin 2.0 enforces stricter NonNull requirements, causing tests related
to `SurfaceControl.Transaction#setPosition` to fail.
These tests previously passed with Kotlin 1.9 without explicitly
handling the return value of `setPosition`, which now requires returning
a NonNull object.

This commit fixes the failing tests by explicitly returning the mocked
`Transaction` object when `setPosition` is called. And updated the
source code to use getLeash instead of directly getting `mTaskSurface`
in order to hijack the `setPosition` call.

Bug: 391717499
Test: atest
Flag: EXEMPT unblock presubmit
Change-Id: I081b6e7695aaba6d745d559b548eaa8b66b51d1f