Update README, Dockerfiles to pass build, promote smoother on-boarding

aguestuserrequested to merge
aguestuser/bitmask_android:develop into develop

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

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