runOnAndroidMainThread(): Don't block QThreadPool::globalInstance() with timout awaiters

A waiting task on QThreadPool::globalInstance() will block the worker
thread that it was scheduled on, making it unavailable for productive
work. That's why one should only put CPU-bound tasks onto
QThreadPool::globalInstance(). When blocking nonetheless, use the
releaseThread()/reserveThread() trick to avoid deadlocks caused by the
pool running out of workers.

So, do that here.

Task-number: QTBUG-109586
Change-Id: Ia2660c69e1f23b5df0c308576301aac6e05d4725
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Assam Boudjelthia <assam.boudjelthia@qt.io>
(cherry picked from commit c0496013484c35ab9b1a29ffb0f1eb687ef6db78)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/corelib/platform/android/qandroidnativeinterface.cpp

@@ -176,6 +176,14 @@ QFuture<QVariant> QNativeInterface::QAndroidApplication::runOnAndroidMainThread(
                 loop.quit();
             });
             watcher.setFuture(future);
+
+            // we're going to sleep, make sure we don't block
+            // QThreadPool::globalInstance():
+
+            QThreadPool::globalInstance()->releaseThread();
+            const auto sg = qScopeGuard([] {
+               QThreadPool::globalInstance()->reserveThread();
+            });
             loop.exec();
         });
     }