Use the same transaction to set position of the leash

When a leash of an insets source is being created, it's initial position
would be set in InsetsSourceProvider.ControlAdapter#startAnimation. And
then, when the position changes, the new position would be set in
InsetsSourceProvider#onPostLayout. Both places should use the same
transaction. Otherwise, there is no guarantee which transaction will be
applied first.

Fix: 282880421
Test: Fold and unfold a foldable device. See if taskbar stays at bottom.
Change-Id: Ib959e91dd25bf1ddb34b25aecda45ffcd396c4c6