Clear spiedInstance field for spyOn objects.

All spyOn instances are directly returned as the mock, and put in the
mock map with a strong ref in the value of the map. Due to the strong
ref in the value the mock map won't purge that item.

Note this is not a complete solution, for the same reason as its parent
commit. MockCreationListner is registered to a ThreadLocal, so we can
only track mocks created in the same thread where it's registered. That
includes all befores and afters, but we basically lose all mocks created
in test cases because they're run with timeout in a different thread.
They are mostly ActivityRecords.

Bug: 123984854
Test: Smaller memory pressure shown in mem dump.
Change-Id: If3c488a23ab9c59a63d9844fc995e4bb0313896a