Bug#814149: add alternative for /usr/bin/dx

[Hans-Christoph Steiner]

Seems to me that if y'all don't want to add an alternatives setup, then
we'll just do a Conflicts.  All of the Android docs and tools just
reference 'dx' so not providing 'dx' is not an option.  Plus in my 35
years of using computers in dotcom startups, media and art, psychiatry,
mobile and desktop app development, and digital security, I've never
heard of OpenDX, so I think these two domains very rarely overlap.

.hc