Update README, Dockerfile to build, smoothing on-boarding

Closes leap/bitmask_android!1



README:
add detailed instructions on installing and compiling
provide workarounds for gotchas setting up emulator on 64-bit debian
provide updated dockerfile & offer instructions for building in docker
Docker:
move dockerfiles to registry contained w/in bitmask_android repo
provide separate dockerfiles for sdk (base), ndk, and emulator images
resolve issues causing containerized build to fail, including:
upstream ics-openvpn conflict with clang (:. transitively with ndk >
r12b)
(subtle!) missing dependency of ndk r12b on file (message: "missing
file" meant a missing program called "file" not an actual missing file)
missing locales causing errors in copyIcsOpenVPNFiles build step
(resolved by generating all locales in dockerfile)
silent failure in sdknmanager's isntallation of platforms;android-23
during image build causing install failure in remote (but not local!)
containers

Signed-off-by: kwadronaut <kwadronaut@leap.se>