Fix assertion failure in TaskPositionerTest

The assertion error is caused because the test tries to shrink window
width more than mMinVisibleWidth.

The CL reduces the amount of mouse move so that it does not shrink the
window beyond mMinVisibleWidth.

Bug: 70123691
Test: atest TaskPositionerTest#testPortraitPreservedWindowResizingDragTopLeft
Change-Id: I03b8b712e365bf07a6b9112d80d4a5e0f2ea77c8