We can't use Parcel.writeValue() to write the ParcelFileDescriptor, otherwise
it leaks when returning the value to the caller (the flag gets lost). Change
the way DropBoxManager.Entry gets serialized so that it uses a bit of its own
flags value to track whether the data is a byte[] or a ParcelFileDescriptor.
Modify the dropbox unit test to add extensive checking of Entry serialization
and deserialization under various circumstances, and to include a regression
test to ensure that FD leaking doesn't happen.
Bug: 2847738
Change-Id: I4ccd17dd03ffab234340cd359e6f3510fdf81193
6e6d60d4c8