Reset PiP shadow radius in onTaskVanished

This is a follow-up of ag/17527240 in which we do not cover the case
that a Task in pinned mode is directly removed via Task#moveTaskToBack

Video: http://recall/-/aaaaaabFQoRHlzixHdtY/cMG5Uh1kLC7EyAs2EbDVkM
Bug: 228793904
Test: follow the reproduce step in bug, see video
Test: atest WMShellUnitTests:PipTaskOrganizerTest
Change-Id: Ib7472908a81d56d34216ca272afbd9f1a2668b89