Add onCleanup() method to Pluggables

onCleanup() is called on all pluggables at the end of each pipeline run.
This allows pluggables to clean up any temporary state they may have
created during the run.

We'll use this in a following CL to reset a cache in the
PreparationCoordinator's filter at the end of every run.

Test: atest SystemUITests
Change-Id: I9567aaf50b55aae4241811404856d4225872e972