Cast: Start a session when a Cast route is selected

[peterhel]

Cast: start a session when a Cast route is selected

Problem

Apps using the modern Cast SDK (play-services-cast-framework) — Prime Video, Netflix,
Disney+, YouTube, and any app built on CastContext/SessionManager — cannot start a
Cast session through microG. The Cast button discovers and lists Chromecasts (route
discovery works), but selecting a device does nothing: no session starts, no connection is
attempted, and the app's SessionManagerListener never fires. On the device this looks
like a silent failure (the symptom reported for Prime Video on /e/OS).

Root cause

CastContextImpl builds the merged MediaRouteSelector and holds the app's IMediaRouter
and its ISessionProviders, but it never registers a route-selection callback.
MediaRouterCallbackImpl — which already contains the correct "on selection, start the
session" logic — is imported by CastDynamiteModuleImpl but never instantiated or
registered with the router. As a result nothing observes route selection, so the session
lifecycle is never driven.

A second, latent bug sits in the provider lookup: defaultSessionProvider is fetched with
sessionProviders.get(categoryForCast(appId)), but the provider map is keyed by the full
control-category string, which carries extra namespace/flag suffixes
(e.g. …/CC1AD845///ALLOW_IPV6). The exact-key lookup misses, leaving
defaultSessionProvider == null, so even once selection is observed the session can't be
created (and MediaRouterCallbackImpl.onRouteSelected would NPE on it).

Fix

play-services-cast-framework only — two files, no API changes:

1. CastContextImpl — register MediaRouterCallbackImpl with the app's
   IMediaRouter (registerMediaRouterCallbackImpl + addCallback) in the constructor, so
   route selection is observed and starts a session. This deliberately goes through the
   IMediaRouter binder rather than touching androidx MediaRouter directly: the dynamite
   runs with the app's Resources but microG's resource IDs, so a direct
   MediaRouter.getInstance(context) from the dynamite throws
   Resources$NotFoundException. The app's MediaRouterProxy already runs androidx
   MediaRouter correctly in-process.
2. CastContextImpl — fall back to a category-prefix match when the exact
   categoryForCast(appId) key isn't present, so the default session provider resolves
   despite the flag suffixes.
3. MediaRouterCallbackImpl.onRouteSelected — null-safety hardening (skip cleanly if
   there is no provider / the unwrapped object isn't a SessionImpl, and fall back to the
   route's own extras when getRouteInfoExtrasById returns null).

Verification

Built into a vtmDefault GmsCore APK and tested against Google's real
play-services-cast-framework (a minimal sender app) on:

• an AOSP Android 14 emulator, and
• a Pixel 2 on LineageOS 22.2 (Android 15).

Before: selecting the Cast route only logs unimplemented Method: onSelect; nothing else
happens.

After: selecting the route drives the full session lifecycle —

MediaRouterCallbackImpl.onRouteSelected → SessionImpl.start → SessionManager.onSessionStarting
CastSender: onSessionStarting   (app's SessionManagerListener fires)
castState: NO_DEVICES_AVAILABLE → CONNECTING

i.e. the session now starts and proceeds to the device-connection/authentication handshake.

Note: completing the connection to a live device additionally requires microG signature
spoofing to be active on the device (so the app's GMS authenticity check passes) — that is
an existing environmental requirement of microG, independent of this change. On a device
without working signature spoofing the session correctly stalls at CONNECTING with a
SERVICE_INVALID from the app's GoogleApiManager; this patch is what gets the session to
that point in the first place.

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