[Android] Implements OS upgrade check and rescheduling

components/background_task_scheduler/android/java/src/org/chromium/components/background_task_scheduler/BackgroundTaskScheduler.java

Index: components/background_task_scheduler/android/java/src/org/chromium/components/background_task_scheduler/BackgroundTaskScheduler.java
diff --git a/components/background_task_scheduler/android/java/src/org/chromium/components/background_task_scheduler/BackgroundTaskScheduler.java b/components/background_task_scheduler/android/java/src/org/chromium/components/background_task_scheduler/BackgroundTaskScheduler.java
index 2fd58ac3812c438dffc583864270e6bc83ca27ca..e690273845cec8b60d884414c42f4836b2c5325b 100644
--- a/components/background_task_scheduler/android/java/src/org/chromium/components/background_task_scheduler/BackgroundTaskScheduler.java
+++ b/components/background_task_scheduler/android/java/src/org/chromium/components/background_task_scheduler/BackgroundTaskScheduler.java
@@ -95,6 +95,41 @@ public class BackgroundTaskScheduler {
         mSchedulerDelegate.cancel(context, taskId);
     }
 
+    /**
+     * Checks whether OS was upgraded and triggers rescheduling if it is necessary.
+     * Rescheduling is necessary if type of background task scheduler delegate is different for a
+     * new version of the OS.
+     *
+     * @param context the current context.
+     */
+    public void checkForOSUpgrade(Context context) {
+        int oldSdkInt = BackgroundTaskSchedulerPrefs.getLastSdkVersion();
+        int newSdkInt = Build.VERSION.SDK_INT;
+        // No OS upgrade detected.
+        if (oldSdkInt == newSdkInt) return;
+
+        // Save the current SDK version to preferences.
+        BackgroundTaskSchedulerPrefs.setLastSdkVersion(newSdkInt);
+
+        // Check for OS upgrades forcing delegate change or "just in case" rescheduling.
+        if (!osUpgradeChangesDelegateType(oldSdkInt, newSdkInt)) return;
+
+        // Explicitly create and invoke old delegate type to cancel all scheduled tasks.
+        // All preference entries are kept until reschedule call, which removes then then.
+        BackgroundTaskSchedulerDelegate oldDelegate =
+                BackgroundTaskSchedulerFactory.getSchedulerDelegateForSdk(oldSdkInt);
+        Set<Integer> scheduledTaskIds = BackgroundTaskSchedulerPrefs.getScheduledTaskIds();
+        for (int taskId : scheduledTaskIds) {
+            oldDelegate.cancel(context, taskId);
+        }
+
+        reschedule(context);
+    }
+
+    /**
+     * Reschedules all the tasks currently scheduler through BackgroundTaskSheduler.
+     * @param context the current context.
+     */
     public void reschedule(Context context) {
         Set<String> scheduledTasksClassNames = BackgroundTaskSchedulerPrefs.getScheduledTasks();
         BackgroundTaskSchedulerPrefs.removeAllTasks();
@@ -108,4 +143,8 @@ public class BackgroundTaskScheduler {
             task.reschedule(context);
         }
     }
+
+    private boolean osUpgradeChangesDelegateType(int oldSdkInt, int newSdkInt) {
+        return oldSdkInt < Build.VERSION_CODES.M && newSdkInt >= Build.VERSION_CODES.M;
+    }
 }