Avoid calling getTopMostTask() twice in Task#fillTaskInfo()

getTopMostTask() is a expensive recursive call that we know won't change
result in a single fillTaskInfo() call, so let's stop calling it twice
in a request.

Bug: 174209413
Test: Builds.
Change-Id: I8239f6e812d41d636573d33f2053bf361db3fc8d