Add log and return if invalid leash onThemeChange

We are seeing reports on PipTransitionState#isInPip()
being bypassed onThemeChanged() despite the state=undefined
with getPinnedTaskLeash returning null, causing an NPE when
applying box shadows.

As a preliminary measure, we are adding a wtflog and returning
early to avoid crashes and to detect future cases during real time
usage.

Bug: 433316431
FLAG: EXEMPT bugfix
Test: N/A
Change-Id: I48273bb6eb87435fe6539f1b23398cc5fb43ef13