Keep main Qt event loop running when Android app is in background

* Process Syncthing events and possibly update Android notifications
* React to network connection changes (metered vs. not metered) while the
  app is in background

This might not be ideal:

* According to the documentation it'll lead to crashes when the app tries
  to draw while in background. I didn't run into any problems after a brief
  test so far, though.
* UI code will run unneccassarily while the app is in the background. The
  code is already unassinging the main thread affinity of models to reduce
  this. It is unfortunately not possible to move the QML engine due to its
  bindings (`QObject::moveToThread: Can not move objects that contain
  bindings or are used in bindings to a new thread.`).

tray/gui/quick/app.cpp

@@ -101,6 +101,7 @@ App::App(bool insecure, QObject *parent)
     , m_sortFilterDevModel(&m_devModel)
     , m_changesModel(m_connection)
     , m_faUrlBase(QStringLiteral("image://fa/"))
+    , m_uiObjects({&m_dirModel, &m_sortFilterDirModel, &m_devModel, &m_sortFilterDevModel, &m_changesModel})
     , m_iconSize(16)
     , m_tabIndex(-1)
     , m_importExportStatus(ImportExportStatus::None)
@@ -698,14 +699,21 @@ void App::handleNewErrors(const std::vector<Data::SyncthingError> &errors)
 void App::handleStateChanged(Qt::ApplicationState state)
 {
     if (m_isGuiLoaded && ((state == Qt::ApplicationSuspended) || (state & Qt::ApplicationHidden))) {
-        qDebug() << "App considered suspended/hidden, reduce polling, stopping UI if requested";
+        qDebug() << "App considered suspended/hidden, reducing polling, stopping UI processing";
         setCurrentControls(false);
+        for (auto *const uiObject : m_uiObjects) {
+            uiObject->moveToThread(nullptr);
+        }
         if (m_unloadGuiWhenHidden) {
             unloadMain();
         }
     } else if (state & Qt::ApplicationActive) {
-        qDebug() << "App considered active, ensuring UI is loaded";
+        qDebug() << "App considered active, continuing polling, resuming UI processing";
         setCurrentControls(true);
+        auto *const uiThread = thread();
+        for (auto *const uiObject : m_uiObjects) {
+            uiObject->moveToThread(uiThread);
+        }
         if (!m_isGuiLoaded) {
             deletePipelineCache();
             loadMain();

tray/gui/quick/app.h

@@ -35,6 +35,7 @@
 #include <QJniObject>
 #endif
 
+#include <array>
 #include <optional>
 
 QT_FORWARD_DECLARE_CLASS(QTextDocument)
@@ -328,6 +329,7 @@ private Q_SLOTS:
     QJsonObject m_settings;
     QString m_faUrlBase;
     std::optional<QString> m_status;
+    std::array<QObject *, 5> m_uiObjects;
     QString m_log;
     int m_iconSize;
     int m_tabIndex;

tray/resources/AndroidManifest.xml.in

@@ -57,6 +57,10 @@
             <meta-data
                 android:name="android.app.splash_screen_drawable"
                 android:resource="@drawable/splash" />
+            <!-- keep main Qt event loop running when app is in background -->
+            <meta-data
+                android:name="android.app.background_running"
+                android:value="true" />
         </activity>
         <service android:name=".SyncthingService" android:foregroundServiceType="specialUse" android:exported="true">
             <property android:name="android.app.PROPERTY_SPECIAL_USE_FGS_SUBTYPE" android:value="Synchronizes files continuously in the background."/>