Refactor android-arm docker image

[Dim0N22]

This PR introduces a couple of improvements:

• Updated base image
• Unified image tags (elementumorg/cross-compiler:base instead of cross-compiler:base)
• Dockerfile uses multistage builds to improve performance and minimize resulting image
• Resulting image is approx. 3 times smaller (900mb instead of 2700mb)

I've tested it with ElementumOrg/libtorrent-go and build succeeded.

This is a draft PR, only android-arm is refactored, but I want to work on all other images as well.
Let me know what you think.

[elgatito]

@Dim0N22 Debian stretch is used, because newer OS requires newer glibc, so binaries won't run on devices with Kernel 3.14, which is used in many media players.