Replace AndroidAsync WebSocket transport with OkHttp

[mzorz]

What

Replaces the realtime WebSocket transport from AndroidAsync (com.koushikdutta.async:androidasync:3.1.0) to OkHttp.

AndroidAsync is unmaintained (last release ~2022) and the source of intermittent transport crashes in the field — notably the java.lang.AssertionError: double connect callback during the SSL handshake (AsyncHttpClient$3.onConnectCompleted). OkHttp is actively maintained and already a dependency of most host apps (Simplenote ships OkHttp 5.x), so this removes a dead dependency without adding real weight.

Changes

• OkHttpWebSocketProvider (new) — implements WebSocketManager.ConnectionProvider, mapping Connection{close,send} / ConnectionListener{onConnect,onMessage,onDisconnect,onError} onto OkHttp's WebSocketListener. OkHttp's send/close are thread-safe, so it drops the main-looper Handler hop the AndroidAsync provider needed.
• onFailure routing — after a successful upgrade, OkHttp's onFailure is a terminal disconnect (no onClosed follows), so it's routed through listener.onDisconnect() (notifies channels, cancels the heartbeat, schedules reconnect), matching the AndroidAsync end/closed-callback behavior. A pre-open failure goes to onError.
• User-Agent sanitization — Simperium passes the session id as the User-Agent header; OkHttp validates header values strictly (AndroidAsync did not), so non-printable/non-ASCII chars are stripped.
• AndroidClient.buildChannelProvider switched to the new provider. AsyncWebSocketProvider is kept in the tree for now (AndroidAsync is still used for the auth HTTP client).

Validation

• Compiles against the existing ConnectionProvider interface (:Simperium:compileDebugJavaWithJavac).
• OkHttp delivers large inbound messages fully reassembled (16 KB → 2 MB verified via a MockWebServer probe), removing the AndroidAsync large-frame instability.
• Exercised end-to-end in Simplenote Android (composite build): connects (HTTP 101), authenticates, indexes, downloads entity bodies, and recovers cleanly from transient Software caused connection abort disconnects via the new onFailure routing.

Before merge (open items)

• Cert pinning. configureSSL() applies a custom SSLContext + PinningTrustManager to the AndroidAsync mHttpClient. The new OkHttpClient is a plain client and does not replicate it. Port the pinning to the OkHttp builder (sslSocketFactory + X509TrustManager, or CertificatePinner) — or make a deliberate decision to drop the (ancient AndroidPinning 1.0.0) pinning. Flagged with a TODO in AndroidClient.
• Align the OkHttp version with the host app (this PR pins 4.12.0 for toolchain safety; Simplenote ships 5.1.0).
• Optional: once the auth client is also migrated, AndroidAsync can be removed entirely.

Context

This came out of investigating large notes not syncing to Android (Simplenote SIMPL-58). That turned out to be a server-side object-size cap (the server returns 413 EXCEEDS_MAX_SIZE), not a transport bug — so this OkHttp swap is a maintenance/stability improvement, not the fix for that issue. It's worth doing on its own merits (kills the unmaintained, crash-prone transport).