standardize on IocVfs.isVfsUri(Uri) for testing VFS URLs

Everywhere that IocVfs.isVfsScheme() was used, it was operating on a Uri
instance.
send contacts that are shared to ChatSecure

This reads the ContentProvider into a VCard in the VFS, then sends it using
OTRDATA.  It also eliminates another mystery blank, white screen in
response to sharing something to ChatSecure.