refactor(auto): extract Android Auto into feature:auto module

- Move MeshtasticCarAppService, CarSession, CarScreen from app/auto
  to the new :feature:auto module (meshtastic.android.library)
- Move automotive_app_desc.xml → auto_app_desc.xml (respects resourcePrefix)
- Move manifest entries (service + meta-data) into feature module so they
  merge into app rather than living in the app manifest directly
- Fix HostValidator: use ApplicationInfo.FLAG_DEBUGGABLE instead of
  BuildConfig.DEBUG (library modules don't ship their own BuildConfig)
- Fix stale unread counts: replace point-in-time getUnreadCount() with
  flatMapLatest + per-conversation getUnreadCountFlow() so the car screen
  invalidates on new messages, not just topology changes
- Fix ConversationShortcutManager: replace removeAllDynamicShortcuts +
  addDynamicShortcuts with pushDynamicShortcut per conversation to
  preserve usage/ranking history; remove stale shortcuts individually;
  respect getMaxShortcutCountPerActivity() limit
- Fix SHORTCUT_CATEGORY_CONVERSATION: constant lives on ShortcutInfo,
  not ShortcutManagerCompat
- Remove androidx.car.app dependency from :app (now owned by :feature:auto)
- Add :feature:auto to settings.gradle.kts and app dependencies

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

app/src/main/AndroidManifest.xml

@@ -153,25 +153,6 @@
             android:name="google_analytics_default_allow_analytics_storage"
             android:value="false" />
 
-        <!-- Android Auto: declare as a messaging/communications app -->
-        <meta-data
-            android:name="com.google.android.gms.car.application"
-            android:resource="@xml/automotive_app_desc" />
-
-        <!-- Android Auto Car App Service for browsable messaging UI -->
-        <service
-            android:name="org.meshtastic.app.auto.MeshtasticCarAppService"
-            android:exported="true">
-            <intent-filter>
-                <action android:name="androidx.car.app.CarAppService" />
-                <category android:name="androidx.car.app.category.MESSAGING" />
-            </intent-filter>
-        </service>
-
-        <meta-data
-            android:name="androidx.car.app.minCarApiLevel"
-            android:value="1" />
-
         <!-- This is the public API for doing mesh radio operations from android apps -->
         <service
             android:name="org.meshtastic.core.service.MeshService"