Actually keep the phone's sessions alive when the app is backgrounded

The foreground service existed, and four defects in its wiring meant it
mostly did not run. A shell opening was never announced to it — only the
ending was — so the service never came up for a shell at all. An idle
connected Files session counted as nothing. Every refresh restarted the
service, which Android 12+ answers with a crash the moment the app is
backgrounded — a transfer finishing in the pocket took the remaining
connections with it. And POST_NOTIFICATIONS was declared but never
requested, so on Android 13+ the receipt was silently invisible.

Updates while backgrounded now go through the notification manager; a
foregrounded refresh still prefers a real start, so a stop still in
flight cannot leave an orphan receipt over an unprotected process.

src/DodoSSH.Client.Android/Properties/AndroidManifest.xml

@@ -12,7 +12,12 @@
   <uses-permission android:name="android.permission.FOREGROUND_SERVICE" />
   <uses-permission android:name="android.permission.FOREGROUND_SERVICE_DATA_SYNC" />
 
-  <!-- The service's persistent notification. Runtime-requested on API 33+, and refusal is survivable. -->
+  <!--
+    The service's persistent notification. Requested on API 33+ from SessionForegroundService.Reconcile,
+    the first time in this process there is actually something to show — not at launch, where the ask
+    would justify nothing on screen yet. Refusal is survivable: the service still starts and still holds
+    the process in the foreground either way, so a "no" costs the notification and nothing else.
+  -->
   <uses-permission android:name="android.permission.POST_NOTIFICATIONS" />
 
   <!-- Releases the device key. See AndroidDeviceKeyStore. -->