MemoryIntArray: track the owned file descriptor in a PFD.

AUPT is triggering an fdsan diagnostic when creating a temporary
ParcelFileDescriptor to write to a Parcel. This code seems correct at
first glance, so under the assumption that some other code is closing
the file descriptor out from under us, keep our owned file descriptor
around as a ParcelFileDescriptor to catch the perpetrator in the act.

Bug: http://b/112405224
Test: atest MemoryIntArrayTest
      (testAshmemSizeMatchesMemoryIntArraySize failed/crashed before, fails now)
Change-Id: Ie8ff7562c78ecde4cf1757d572ecb733213cc975