Fix wrong touch region of split sreen

To compatable with Ia36d7529 which introduced a single-top root task for
multi-window split, updates space-to fill bounds of letter box to the
task level.

Fix: 227563144
Test: Test: atest WmTests:SizeCompatTests
Test: verify touch region with dumpsys input in multi-window split
Change-Id: I64375f66ce06e47c6c5421d2cd30c1d63f5a47f8