BaseParceledListSlice: remove List reference once transfer completes

Currently, BaseParceledListSlice will hold a reference to the List<>
until the Binder object is destroyed. However, this requires that the
receiver run GC before the sender's global Binder reference can be
removed, causing the List to be retained until both receiver and
sender perform GC. Since the List is usually the large part of the
object, instead clear the reference to the List once the transfer to
the receiver completes, allowing the List to be GC'd before the
receiver GC's its reference to the sender's Binder object.

Test: boots
Bug: 213236807
Change-Id: I22d6e56c953db3c85179911b909d5b6e7c8ba784