Don't set a task description for work lock overlay

Just use the one in the previous activity on the task stack, it's less
code and also has the benefit that if anything new happens in that
activity, all of the observers will be notified about the new
description straight away.

Bug: 29244275
Test: adb shell service call trust 6 i32 10 i32 1 && adb shell am start -a com.android.systemui.recents.TOGGLE_RECENTS
Change-Id: I56d3b627ccda82cd2a2e520de0e170e4fa4acc77