[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/client.go

@@ -113,8 +113,9 @@ type Client struct {
 
 	// The file drop handle survives engine restarts so the UI keeps one listener
 	// registration and one history view across reconnects. See fileDropFor.
-	fileDropMu sync.Mutex
-	fileDrop   *FileDrop
+	fileDropMu   sync.Mutex
+	fileDrop     *FileDrop
+	fileDropSink FileDropSink
 }
 
 func (c *Client) setState(cfg *profilemanager.Config, cacheDir string, cfgPath string, cc *internal.ConnectClient) {