[client] Stage incoming file drop payloads through a platform sink

Android cannot address the user's shared storage by path, so a received
file had to land in app-private storage and be copied out afterwards,
needing twice the space of the transfer.

Put the staging area behind a Sink interface the receiver writes every
payload through. The filesystem spool implements it unchanged and stays
the default; a platform that cannot be addressed by path implements the
gomobile-bound half instead and stages payloads wherever it can reach.

The writer reports its own total rather than returning a written count:
gomobile copies a []byte argument into a fresh Java array and carries no
count back out.

A failed delivery now drops the staged payloads. The filesystem spool
swept them up on its next pass, but a sink holding entries the engine
cannot address by path has no such fallback.

client/internal/filedrop/spool.go

@@ -61,8 +61,10 @@ func (s *Spool) Received(id OfferID, index int) (int64, error) {
 	return info.Size(), nil
 }
 
-// Write appends the payload at offset, truncating any bytes past it first.
-func (s *Spool) Write(id OfferID, index int, offset int64, r io.Reader, limit int64) (int64, error) {
+// Write appends the payload at offset, truncating any bytes past it first. The
+// announced name plays no part: payloads are staged under their index and only
+// take their name in Deliver.
+func (s *Spool) Write(id OfferID, index int, _ string, offset int64, r io.Reader, limit int64) (int64, error) {
 	if offset < 0 {
 		return 0, fmt.Errorf("negative offset %d", offset)
 	}
@@ -92,6 +94,12 @@ func (s *Spool) Write(id OfferID, index int, offset int64, r io.Reader, limit in
 	return offset + written, nil
 }
 
+// Deliver moves an offer's staged payloads into destDir under their announced
+// names and returns where each one landed.
+func (s *Spool) Deliver(offer Offer, destDir string) ([]string, error) {
+	return deliver(s, offer, destDir)
+}
+
 // Path returns the staged path of one item for the platform layer to deliver from.
 func (s *Spool) Path(id OfferID, index int) string {
 	return s.filePath(id, index)