Fix null snapshot issue

If there is no snapshot, don't play convert task animation (as that
requires a snapshot). When PIP2 is disabled and converting a task to a
bubble, the incoming transition has a change with mode CHANGE. But it
does not have a snapshot to animate. We always expect a change to have
the snapshot. Add a check that the snapshot does exist, before trying to
play a convert animation. If there is no snapshot, play an open
animation.

Fixes: 419090304
Test: disable PIP2 flags, play a Youtube video in PIP, long-press on
Youtube and launch it in a Bubble, check that PIP hides and Youtube
opens in a Bubble
Flag: com.android.wm.shell.enable_create_any_bubble

Change-Id: Ieb7446facaba958efdbcae81c2d968aa0626b2f1