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...)

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

@@ -79,6 +79,7 @@ public class RNFetchBlobReq extends BroadcastReceiver implements Runnable {
     }
 
     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 class RNFetchBlobReq extends BroadcastReceiver implements Runnable {
             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 class RNFetchBlobReq extends BroadcastReceiver implements Runnable {
                 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 class RNFetchBlobReq extends BroadcastReceiver implements Runnable {
     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 class RNFetchBlobReq extends BroadcastReceiver implements Runnable {
             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);