Avoid leaking file descriptors when returning drop box events.

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