Add ability to cancel android DownloadManager fetches (#502)

This just requires a bit of bookkeeping that keeps track of the task ID to the download manager ID.
Note that the behavior of the download manager remove method is to remove the download and the downloaded file, whether partial or complete.
https://developer.android.com/reference/android/app/DownloadManager.html#remove(long...)

Author: wschurman

android/src/main/java/com/RNFetchBlob/RNFetchBlobReq.java

@@ -79,6 +79,7 @@ enum ResponseFormat {
     }
 
     public static HashMap<String, Call> taskTable = new HashMap<>();
+    public static HashMap<String, Long> androidDownloadManagerTaskTable = new HashMap<>();
     static HashMap<String, RNFetchBlobProgressConfig> progressReport = new HashMap<>();
     static HashMap<String, RNFetchBlobProgressConfig> uploadProgressReport = new HashMap<>();
     static ConnectionPool pool = new ConnectionPool();
@@ -135,6 +136,13 @@ public static void cancelTask(String taskId) {
             call.cancel();
             taskTable.remove(taskId);
         }
+
+        if (androidDownloadManagerTaskTable.containsKey(taskId)) {
+            long downloadManagerIdForTaskId = androidDownloadManagerTaskTable.get(taskId).longValue();
+            Context appCtx = RNFetchBlob.RCTContext.getApplicationContext();
+            DownloadManager dm = (DownloadManager) appCtx.getSystemService(Context.DOWNLOAD_SERVICE);
+            dm.remove(downloadManagerIdForTaskId);
+        }
     }
 
     @Override
@@ -172,6 +180,7 @@ public void run() {
                 Context appCtx = RNFetchBlob.RCTContext.getApplicationContext();
                 DownloadManager dm = (DownloadManager) appCtx.getSystemService(Context.DOWNLOAD_SERVICE);
                 downloadManagerId = dm.enqueue(req);
+                androidDownloadManagerTaskTable.put(taskId, Long.valueOf(downloadManagerId));
                 appCtx.registerReceiver(this, new IntentFilter(DownloadManager.ACTION_DOWNLOAD_COMPLETE));
                 return;
             }
@@ -438,6 +447,8 @@ public void onResponse(Call call, Response response) throws IOException {
     private void releaseTaskResource() {
         if(taskTable.containsKey(taskId))
             taskTable.remove(taskId);
+        if(androidDownloadManagerTaskTable.containsKey(taskId))
+            androidDownloadManagerTaskTable.remove(taskId);
         if(uploadProgressReport.containsKey(taskId))
             uploadProgressReport.remove(taskId);
         if(progressReport.containsKey(taskId))
@@ -635,6 +646,8 @@ public void onReceive(Context context, Intent intent) {
             Context appCtx = RNFetchBlob.RCTContext.getApplicationContext();
             long id = intent.getExtras().getLong(DownloadManager.EXTRA_DOWNLOAD_ID);
             if (id == this.downloadManagerId) {
+                releaseTaskResource(); // remove task ID from task map
+
                 DownloadManager.Query query = new DownloadManager.Query();
                 query.setFilterById(downloadManagerId);
                 DownloadManager dm = (DownloadManager) appCtx.getSystemService(Context.DOWNLOAD_SERVICE);