Prepare close transition before adjust focusable task.

There will adjust task layer when finish the last activity in a task,
to preventing the layer change transaction being applied by pending
transaction, prepare close transition before adjust focusable task.

Flag: EXEMPT bugfix
Bug: 379854272
Test: launch several tasks in desktop mode, close a task and verify the
task layer won't change before transition ready.

Change-Id: I510e9588fbc2009ae6e61f3c78d537b7fdd26425