Parcel: remove use of getOpenAshmemSize

Parcel native pointers are created when the Parcel is created, and they
are destroyed in finalize. When a Java object is finalized, we shouldn't
be calling methods on this (note - due to the way that compareData is
written, it will throw NPE if otherParcel is null - and if this were not
the case, the use of getOpenAshmemSize here - which will frequently be
zero - would break the comparison here).

Bug: 195752513
Test: atest android.os.ParcelTest android.os.cts.ParcelTest
Change-Id: Iad86624d6e0aad2ca37273ddbcb456dff7f48d46