SharedMemory: break Cleaner reference cycle.

Previously, the Cleaner we create to close the ashmem file descriptor
used a thunk that held a strong reference to the FileDescriptor we
wanted to clean up, which prevented the Cleaner from ever running.

Break the cycle by storing the integer value of the file descriptor
instead.

Bug: http://b/138323667
Test: treehugger
Change-Id: I613a7d035892032f9567d59acb04672957c96011