Provide alternative find-task method without ActivityRecord

Restructure the task looks-up methods to allow finding tasks
without passing in an ActivityRecord. This opens the opportunity
of looking reusable tasks before building an ActivityRecord, which
is the first step of stand-in activity replacement.

Bug: 175354896
Test: presubmit
Change-Id: Ic769790f0e5cbc921e144e1bb3d696519fc13555