Fix Quick Share receiving and duplicate discovery

[hkay-dev]

AI-assisted: Codex helped write and review this code and PR.

Why

The receiver sent ACCEPT before opening its Downloads output stream. The Mac could begin sending before Android was ready, leaving a pending file even when the sender reported success. During testing, duplicate discovery requests also registered a second mDNS service and coincided with the Mac staying on Connecting.

What changed

• Open the Downloads output stream before sending ACCEPT. Require that stream for payload data, check the final byte count, and clear MediaStore's pending flag only for a complete file.
• Start the handshake after the service installs connection callbacks. Failed connections now close and notify the service once, and the active-connection map supports calls from different threads.
• Ignore a duplicate discovery request while discovery is active.

Checks

• :app:assembleDebug passes.
• Two consecutive Mac-to-Android sends completed using a side-by-side Android test build. A later send from the final Mac branch also appeared in /sdcard/Download/, matched the Mac copy by SHA-256, and had is_pending=0 in MediaStore.
• No manifest, settings, build file, dependency, or wire-format changes.

Notes

This changes inbound Quick Share and duplicate discovery only. Other Android file-saving paths are unchanged.

Companion Mac PR: sameerasw/airsync-mac#301