Android: Create new QtThread when needed

When we quit Qt in an Android app, we also quit the Java thread created
to act as the Qt main thread. This works well for normal Qt for Android
apps, where the lifetime of the Qt part and the whole Android app is
essentially the same. However, in QtQuick for Android Qt can be just
a part of the entire app, and the lifecycles are not directly tied.
Hence, we might want to quit Qt when the part of the app that has it
is not active, and restart it when the Qt part of the app comes
active again, and for this we need to create a new QtThread.

Set the QtThread variable to null when it has been terminated,
and if it's required again, create a new instance.

Task-number: QTBUG-130610
Pick-to: 6.8
Change-Id: I430e195fd319a20c9627ed98d6f30858cebc49c4
Reviewed-by: Assam Boudjelthia <assam.boudjelthia@qt.io>
(cherry picked from commit 7c7b63f3e85436125baf2211bc5e10556736241b)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/android/jar/src/org/qtproject/qt/android/QtEmbeddedDelegate.java

@@ -75,9 +75,8 @@ class QtEmbeddedDelegate extends QtActivityDelegateBase
                     m_activity.getApplication().unregisterActivityLifecycleCallbacks(this);
                     QtNative.unregisterAppStateListener(QtEmbeddedDelegate.this);
                     QtEmbeddedViewInterfaceFactory.remove(m_activity);
-                    QtNative.terminateQt();
+                    QtNative.quitQt();
                     QtNative.setActivity(null);
-                    QtNative.getQtThread().exit();
                 }
             }
         });

src/android/jar/src/org/qtproject/qt/android/QtNative.java

@@ -44,7 +44,8 @@ public class QtNative
 
     // a list of all actions which could not be performed (e.g. the main activity is destroyed, etc.)
     private static final BackgroundActionsTracker m_backgroundActionsTracker = new BackgroundActionsTracker();
-    private static final QtThread m_qtThread = new QtThread();
+    private static QtThread m_qtThread = null;
+    private static final Object m_qtThreadLock = new Object();
     private static ClassLoader m_classLoader = null;
 
     private static final Runnable runPendingCppRunnablesRunnable = QtNative::runPendingCppRunnables;
@@ -193,7 +194,15 @@ public class QtNative
     }
 
     static QtThread getQtThread() {
+        if (m_qtThread != null)
             return m_qtThread;
+
+        synchronized (m_qtThreadLock) {
+            if (m_qtThread == null)
+                m_qtThread = new QtThread();
+
+            return m_qtThread;
+        }
     }
 
     interface AppStateDetailsListener {
@@ -339,12 +348,13 @@ public class QtNative
 
     static void startApplication(String params, String mainLib)
     {
-        m_qtThread.run(() -> {
+        QtThread thread = getQtThread();
+        thread.run(() -> {
             final String qtParams = mainLib + " " + params;
             if (!startQtAndroidPlugin(qtParams))
                 Log.e(QtTAG, "An error occurred while starting the Qt Android plugin");
         });
-        m_qtThread.post(QtNative::startQtApplication);
+        thread.post(QtNative::startQtApplication);
         waitForServiceSetup();
         m_stateDetails.isStarted = true;
         notifyAppStateDetailsChanged(m_stateDetails);
@@ -363,6 +373,17 @@ public class QtNative
         });
     }
 
+    static void quitQt()
+    {
+        terminateQt();
+        m_stateDetails.isStarted = false;
+        notifyAppStateDetailsChanged(m_stateDetails);
+        getQtThread().exit();
+        synchronized (m_qtThreadLock) {
+            m_qtThread = null;
+        }
+    }
+
     @UsedFromNativeCode
     static int checkSelfPermission(String permission)
     {