Ensure SerialExecutor has no more pending tasks.

* We need to ensure this is the case because we might shutdown
  SystemAlarmService without actually completing all the tasks.

Test: Existing unit tests pass. Ran integration tests.
Fixes: b/138238197
Change-Id: I72fe2ddb1607995721eae320ce41accad93fee8f

diff --git a/work/workmanager/src/main/java/androidx/work/impl/background/systemalarm/SystemAlarmDispatcher.java b/work/workmanager/src/main/java/androidx/work/impl/background/systemalarm/SystemAlarmDispatcher.java
index 548da36..5ddb288 100644
--- a/work/workmanager/src/main/java/androidx/work/impl/background/systemalarm/SystemAlarmDispatcher.java
+++ b/work/workmanager/src/main/java/androidx/work/impl/background/systemalarm/SystemAlarmDispatcher.java

@@ -32,6 +32,7 @@
 import androidx.work.impl.ExecutionListener;
 import androidx.work.impl.Processor;
 import androidx.work.impl.WorkManagerImpl;
+import androidx.work.impl.utils.SerialExecutor;
 import androidx.work.impl.utils.WakeLocks;
 import androidx.work.impl.utils.taskexecutor.TaskExecutor;
 
@@ -221,8 +222,11 @@
                 }
                 mCurrentIntent = null;
             }
+            SerialExecutor serialExecutor = mTaskExecutor.getBackgroundExecutor();
+            if (!mCommandHandler.hasPendingCommands()
+                    && mIntents.isEmpty()
+                    && !serialExecutor.hasPendingTasks()) {
 
-            if (!mCommandHandler.hasPendingCommands() && mIntents.isEmpty()) {
                 // If there are no more intents to process, and the command handler
                 // has no more pending commands, stop the service.
                 Logger.get().debug(TAG, "No more commands & intents.");

diff --git a/work/workmanager/src/main/java/androidx/work/impl/utils/SerialExecutor.java b/work/workmanager/src/main/java/androidx/work/impl/utils/SerialExecutor.java
index e48ef4a..982c865 100644
--- a/work/workmanager/src/main/java/androidx/work/impl/utils/SerialExecutor.java
+++ b/work/workmanager/src/main/java/androidx/work/impl/utils/SerialExecutor.java

@@ -57,6 +57,15 @@
         }
     }
 
+    /**
+     * @return {@code true} if there are tasks to execute in the queue.
+     */
+    public boolean hasPendingTasks() {
+        synchronized (mLock) {
+            return !mTasks.isEmpty();
+        }
+    }
+
     @NonNull
     @VisibleForTesting
     public Executor getDelegatedExecutor() {