Hold the reference to overlay in PipTaskOrganizer.

The animator can finish before the overlay is being removed, so we need
to keep a reference in PipTaskOrganizer and use it there for fade out.

Previously the overlay leash was removed at animation end, causing it
to be removed abruptly without smooth animation.

Note that the animation progress from transition is used to update the
state of the overlay, the fractional fade in is synchronized with it.

Bug: 308003114
Test: before http://recall/-/ekEuGtt9d9HWqkUtAzpHx8/bxJIJ3g282Hl5wriD3HE6F
Test: after http://recall/-/ekEuGtt9d9HWqkUtAzpHx8/hG1H7edsQoUGkrq3WoLhsN
Flag: none

Change-Id: I12589c9699d2e2b6af17875f9bc32ba7f47c6365