Use Dispatchers.Main as default delay source where applicable

It reduces the number of redundant threads in the system and makes time source predictable across Android/JavaFx applications

Fixes #2972

kotlinx-coroutines-core/jvm/src/DefaultExecutor.kt

@@ -4,10 +4,25 @@
 
 package kotlinx.coroutines
 
+import kotlinx.coroutines.internal.*
 import java.util.concurrent.*
 import kotlin.coroutines.*
 
-internal actual val DefaultDelay: Delay = DefaultExecutor
+internal actual val DefaultDelay: Delay = initializeDefaultDelay()
+
+private val defaultMainDelayOptIn = systemProp("kotlinx.coroutines.main.delay", true)
+
+private fun initializeDefaultDelay(): Delay {
+    // Opt-out flag
+    if (!defaultMainDelayOptIn) return DefaultExecutor
+    val main = Dispatchers.Main
+    /*
+     * When we already are working with UI and Main threads, it makes
+     * no sense to create a separate thread with timer that cannot be controller
+     * by the UI runtime.
+     */
+    return if (main.isMissing() || main !is Delay) DefaultExecutor else main
+}
 
 @Suppress("PLATFORM_CLASS_MAPPED_TO_KOTLIN")
 internal actual object DefaultExecutor : EventLoopImplBase(), Runnable {

ui/kotlinx-coroutines-javafx/src/JavaFxDispatcher.kt

@@ -34,22 +34,18 @@ public sealed class JavaFxDispatcher : MainCoroutineDispatcher(), Delay {
 
     /** @suppress */
     override fun scheduleResumeAfterDelay(timeMillis: Long, continuation: CancellableContinuation<Unit>) {
-        val timeline = schedule(timeMillis, TimeUnit.MILLISECONDS, EventHandler {
+        val timeline = schedule(timeMillis, TimeUnit.MILLISECONDS) {
             with(continuation) { resumeUndispatched(Unit) }
-        })
+        }
         continuation.invokeOnCancellation { timeline.stop() }
     }
 
     /** @suppress */
     override fun invokeOnTimeout(timeMillis: Long, block: Runnable, context: CoroutineContext): DisposableHandle {
-        val timeline = schedule(timeMillis, TimeUnit.MILLISECONDS, EventHandler {
+        val timeline = schedule(timeMillis, TimeUnit.MILLISECONDS) {
             block.run()
-        })
-        return object : DisposableHandle {
-            override fun dispose() {
-                timeline.stop()
-            }
         }
+        return DisposableHandle { timeline.stop() }
     }
 
     private fun schedule(time: Long, unit: TimeUnit, handler: EventHandler<ActionEvent>): Timeline =