[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/android/filedrop_sink.go

@@ -0,0 +1,83 @@
+//go:build android
+
+package android
+
+import (
+	"fmt"
+
+	"github.com/netbirdio/netbird/client/internal/filedrop"
+)
+
+// FileDropSink stages incoming payloads on the platform's behalf. Android
+// cannot address the user's shared storage by path, so the writing itself
+// happens in Java and the engine only drives it.
+//
+// The methods mirror filedrop.PlatformSink; see that interface for the contract.
+type FileDropSink interface {
+	// DestinationLabel names where payloads land, for the UI to show in place of
+	// a path the platform does not have.
+	DestinationLabel() string
+	Prepare(offerID string) error
+	Received(offerID string, index int) (int64, error)
+	OpenWriter(offerID string, index int, name string, offset int64, size int64) (FileDropWriter, error)
+	Deliver(offerID string) (string, error)
+	Remove(offerID string)
+	Cleanup(maxAgeSeconds int64)
+}
+
+// FileDropWriter is one payload's destination, opened by a FileDropSink.
+//
+// WriteChunk takes the bytes rather than filling a caller-supplied buffer, and
+// the total is read back through Written: gomobile copies a []byte argument
+// into a fresh Java array and never carries a written count back out.
+type FileDropWriter interface {
+	WriteChunk(p []byte) error
+	Close() error
+	Written() int64
+}
+
+// platformSinkAdapter bridges the gomobile-bound FileDropSink onto the
+// interface the engine consumes. The two differ only in the writer type, which
+// gomobile cannot share between packages.
+type platformSinkAdapter struct {
+	sink FileDropSink
+}
+
+func newPlatformSinkAdapter(sink FileDropSink) filedrop.PlatformSink {
+	return &platformSinkAdapter{sink: sink}
+}
+
+func (a *platformSinkAdapter) DestinationLabel() string {
+	return a.sink.DestinationLabel()
+}
+
+func (a *platformSinkAdapter) Prepare(offerID string) error {
+	return a.sink.Prepare(offerID)
+}
+
+func (a *platformSinkAdapter) Received(offerID string, index int) (int64, error) {
+	return a.sink.Received(offerID, index)
+}
+
+func (a *platformSinkAdapter) OpenWriter(offerID string, index int, name string, offset int64, size int64) (filedrop.PlatformWriter, error) {
+	w, err := a.sink.OpenWriter(offerID, index, name, offset, size)
+	if err != nil {
+		return nil, err
+	}
+	if w == nil {
+		return nil, fmt.Errorf("no destination for %s item %d", offerID, index)
+	}
+	return w, nil
+}
+
+func (a *platformSinkAdapter) Deliver(offerID string) (string, error) {
+	return a.sink.Deliver(offerID)
+}
+
+func (a *platformSinkAdapter) Remove(offerID string) {
+	a.sink.Remove(offerID)
+}
+
+func (a *platformSinkAdapter) Cleanup(maxAgeSeconds int64) {
+	a.sink.Cleanup(maxAgeSeconds)
+}