fix: maintain database may cause anr

Closes lingochamp#1152

Author: Tianhua Ran

library/src/main/java/com/liulishuo/filedownloader/download/CustomComponentHolder.java

@@ -88,7 +88,17 @@ public FileDownloadDatabase getDatabaseInstance() {
         synchronized (this) {
             if (database == null) {
                 database = getDownloadMgrInitialParams().createDatabase();
-                maintainDatabase(database.maintainer());
+                // There is no reusable thread for this action and this action has
+                // a very low frequency, so, a new thread is simplest way to make this action
+                // run on background thread.
+                final String maintainThreadName =
+                        FileDownloadUtils.getThreadPoolName("MaintainDatabase");
+                new Thread(new Runnable() {
+                    @Override
+                    public void run() {
+                        maintainDatabase(database.maintainer());
+                    }
+                }, maintainThreadName).start();
             }
         }
 
@@ -186,7 +196,7 @@ private static void maintainDatabase(FileDownloadDatabase.Maintainer maintainer)
                             || model.getStatus() == FileDownloadStatus.error
                             || (model.getStatus() == FileDownloadStatus.pending && model
                             .getSoFar() > 0)
-                            ) {
+                    ) {
                         // Ensure can be covered by RESUME FROM BREAKPOINT.
                         model.setStatus(FileDownloadStatus.paused);
                     }
@@ -275,6 +285,9 @@ private static void maintainDatabase(FileDownloadDatabase.Maintainer maintainer)
             FileDownloadUtils.markConverted(FileDownloadHelper.getAppContext());
             maintainer.onFinishMaintain();
             // 566 data consumes about 140ms
+            // update by rth: different devices have very large disparity, such as,
+            // in my HuaWei Android 8.0, 67 data consumes about 355ms.
+            // so, it's better do this action in background thread.
             if (FileDownloadLog.NEED_LOG) {
                 FileDownloadLog.d(FileDownloadDatabase.class,
                         "refreshed data count: %d , delete data count: %d, reset id count:"