Set parent on animation-background before building

Setting a parent after building doesn't seem to do much.
For consistency and cleaner traces we'll put the animation
background under the right parent during construction when
it's going under a transition root too.

Change-Id: I2def0ae3391d017a64c118b7a4932b7e25b8d8af
Test: atest FlickerTestsAppLaunch
Test: atest ActivityTransitionTests
Bug: 333525723