Use task top instead of input y coordinate for fullscreen transition.

Uses the y coordinate of the top edge of a task for transition to
fullscreen. This prevents an issue where a transition would not start
due to the drag being on the lower edge of the caption surface.

Bug: 292110228
Test: Manual
Test: atest WMShellUnitTests:FluidResizeTaskPositionerTest
Test: atest WMShellUnitTests:VeiledResizeTaskPositionerTest
Change-Id: I1f2d1cd778372f14da2d7edff6aa7503dfe50c42