Load PDFs in WebView through Google Docs

URLs for the raw PDF are loaded in WebView using Google Docs
embedded viewer.
There is a button on the page to download the file and view
in an external editor.

The alternative to this is to immediately download the file to
the SD card then launch an intent to view the downloaded PDF or
add a button to the action bar to download the file.
This would require additional permissions to write to external storage.
FileGetter class implemented

TODO
    Check if external storage available
    Open intent to view downloaded file using DownloadCompleteReceiver
Open PDF/ZIP files in external viewer

Downloading progress dialog appears.
    Should the progress of the dl be shown?
On back pressed, the download is cancelled.
Uses built in DownloadManager for Gingerbread and above.
For Froyo, use internal googledocs Web viewer.

TODO
    Implement a FileGetter for SDK8 and below.