Create android-arm and android-arm64 images

[vweevers]

See also https://github.com/Level/leveldown/pull/587#issuecomment-457943370. They must have:

ENV GYP_DEFINES target_arch=arm(64) android_target_arch=arm(64) host_os=linux OS=android.

Note:

• IIRC you only need quotes around the value if you use the GYP_DEFINES= form, rather than GYP_DEFINES<space>
• OS must be uppercase
• target_arch might not be necessary, because we're also passing an ARCH (I didn't test without yet)

[ralphtheninja]

We might not need target_arch since prebuildify sets it here https://github.com/prebuild/prebuildify/blob/master/index.js#L124

[ralphtheninja]

https://github.com/prebuild/docker-images/commit/dc3a112ee2c61115d856a687c1d7380bd1884cd2 and https://github.com/prebuild/docker-images/commit/bd98af328403fde7e1037406632268c98603fafe