[android_alarm_manager] Move method channel usage to the platform main thread (#1641)

Fixes https://github.com/flutter/flutter/issues/32976

packages/android_alarm_manager/android/src/main/java/io/flutter/plugins/androidalarmmanager/AlarmService.java

diff --git a/packages/android_alarm_manager/android/src/main/java/io/flutter/plugins/androidalarmmanager/AlarmService.java b/packages/android_alarm_manager/android/src/main/java/io/flutter/plugins/androidalarmmanager/AlarmService.java
index 6c13f79..2851846 100644
--- a/packages/android_alarm_manager/android/src/main/java/io/flutter/plugins/androidalarmmanager/AlarmService.java
+++ b/packages/android_alarm_manager/android/src/main/java/io/flutter/plugins/androidalarmmanager/AlarmService.java
@@ -9,6 +9,7 @@
 import android.content.Context;
 import android.content.Intent;
 import android.content.SharedPreferences;
+import android.os.Handler;
 import android.util.Log;
 import androidx.core.app.JobIntentService;
 import io.flutter.plugin.common.MethodChannel;
@@ -421,7 +422,7 @@
    * callbacks have been executed.
    */
   @Override
-  protected void onHandleWork(Intent intent) {
+  protected void onHandleWork(final Intent intent) {
     // If we're in the middle of processing queued alarms, add the incoming
     // intent to the queue and return.
     synchronized (sAlarmQueue) {
@@ -434,6 +435,13 @@
 
     // There were no pre-existing callback requests. Execute the callback
     // specified by the incoming intent.
-    executeDartCallbackInBackgroundIsolate(intent);
+    new Handler(getMainLooper())
+        .post(
+            new Runnable() {
+              @Override
+              public void run() {
+                executeDartCallbackInBackgroundIsolate(intent);
+              }
+            });
   }
 }