WiFiDirectDemo: fix broken sample code

Sender sent the file indeed, but receiver cannot save content normally due to below reason:
1. App shouldn't create the directory in external storage root,
   it should put files in its application path.
2. It is not allowed using file:// uri for posting file content above API 24,
   it should be done by FileProvider.

Bug: 113998459
Test: manual test with two devices
Change-Id: Ie50dd64206009f07414a85262e725cc4d8ac1757