Update and simplify the Android docker setup

Hey, as discussed on discord, I took a knack at updating the android side of things.

Dockerfile:

• Replace the download ARGs, now we only need the following (and only NDK is required, rest have default values set)

ANDROID_NDK_VERSION
ANDROID_CMD_LINE_TOOLS_VERSION
ANDROID_BUILD_TOOLS_VERSION
ANDROID_PLATFORM_VERSION

The dockerfile now uses the most recent command line tools, and the SDKmanager is used to download all the dependencies!

• updated documentation to reflect the changes
• updated the ci generator to use the new env vars

Closes #81 (closed)