Clean up StartingWindowRecord from record manager once it is removed.

The snapshot starting window can scheduled to be remove later while
calling removeIfPossible, but it should also clean up itself from the
record manager.

Bug: 299852784
Test: create several task snapshot window, check on the dumpheap to
verify them can be removed from record manager.

Change-Id: If704363bb24124b618e7fe8dfeb98bf2c8286058