explicitly use python3 in script, docker and snapcraft

git clone doesn't clones the submodules 'external/cpu_features' and 'external/sdbus-cpp'

Add support for proximity sensorAdd documentation about DBUS interfaceMake Anbox compile with Clang 9.0

apt install python-minimal will setup python 2.7

Added python2 as build dependency

I'm a newbie. I reckon most people on github aren't. But in the off-chance that the rare newbie visits, I think it's good to have a simple statement about Anbox that a newbie could understand. Hence my suggested change.

Fixes #7.