Start animations in default handler after start transaction

This was basically posting animation start onto anim executor
which is a different thread. This caused a race where the
animation could start (and apply some surface changes) before
the transition startTransaction which could cause flicker in
the corners (outside the rounded corner) since the animation
background surface wasn't shown yet.

Bug: 265007895
Test: flicker TaskTransitionTest
Change-Id: I717a4a1e8be1ace1bbbed32b358290d2c59650b6