Add downloadability of files

[jan-jockusch]

Platforms affected

Android only

Motivation and Context

Downloading files is blocked by the InAppBrowser by default. This PR opens up the possibility.

Description

This adds some callback code which opens an ACTION_VIEW intent with a file when the file name extension matches a list of popular extensions.

Testing

Point an InAppBrowser to a PDF URL. Without this patch, the browser does nothing. With this patch, a requester opens to choose the app to open the file with.

Checklist

• I've run the tests to see all new and existing tests pass
• I added automated test coverage as appropriate for this change
• Commit is prefixed with (platform) if this change only applies to one platform (e.g. (android))
• If this Pull Request resolves an issue, I linked to the issue in the text above (and used the correct keyword to close issues using keywords)
• I've updated the documentation if necessary