Unable to open content from Dropbox

# Bug Report
When we try to open a file via an intent from the dropbox application (168.2.2) with the cordova plugin 6.0.1 it fails.

## Problem
The resolveLocalFileSystemURL fails when trying to retrieve the modofied date. this

### What is expected to happen?
We should be able to retrieve the file entry

### What does actually happen?
It fails.

## Information
the problem is that dropbox returns a string of the form
Weekday, Day Month Year Hour:MInutes:Seconds OffsetFrom GMT
Ex:
Mon, 21 Oct 2019 14:49:59 -05:00
which is not able to be parsed as a long.

### Command or Code
The method used is resolveLocalFileSystemURL, which eventually calls the function lastModifiedDateForCursor in contentFileSystem.java

if (columnIndex != -1) {
String dateStr = cursor.getString(columnIndex);
if (dateStr != null) {
return Long.parseLong(dateStr);
}
}

### Environment, Platform, Device
Android Pixel 2

### Version information
Cordova: 7.1
Cordova-plugin-file: 6.0.1
android platform: 6.4.0
Other Frameworks: AngularJS
Android : 10
Dropbox 168.2.2

## Checklist

- [x] I searched for existing GitHub issues
- [ ] I updated all Cordova tooling to most recent version
- [x] I included all the necessary information above

Start in contentFileSystem.java at lastModifiedDateForCursor, reached through resolveLocalFileSystemURL. Reproduce the Android Pixel 2 scenario using a Dropbox file and inspect how its modified-date string is handled. Done means the file entry can be retrieved successfully when Dropbox returns the documented date format.

Written by the indexing model from the issue text.