Delete the delay for keeping tasksnapshot on screen.

Tasksnapshot window can block touch event so user may not able
to touch the screen after showing task snapshot. Remove the delay
so app window can response touch event as soon as possible.

Bug: 187227846
Test: atest TaskSnapshotWindowTest
Change-Id: I5fb0fa3a1e6a5e6210d3baf400a84c5892bd2e34