CB-7487 [Android] Broadcast file write

This allows MTP USB shares to show the file immediately without reboot/manual refresh using 3rd party app.
I agree with what you are saying here.  By the time we are in writeToFileAtURL it seems the file already exists, so I tried moving this logic to getFileForLocalURL.  In here I was checking if the file existed prior to creation, then broadcasting if it was created.  

The results however were strange.  Though the files would appear, they would sometimes be 0 bytes in size.  Trying to debug the javascript portion always resulted in 0 byte files.

I then tried re-testing the broadcast after the write (where it is in this commit) and it seems to work as expected.  I can't really explain why this occurring, perhaps you have some insight on this?  If not, I think it's probably best to leave the broadcast after every write to be safe.