refactor(mk): move build rules into separate scripts (#855)

See https://github.com/ooni/probe/issues/2218

MOBILE/android/setup

@@ -0,0 +1,52 @@
+#!/bin/bash
+
+set -euo pipefail
+
+GOOS=$(go env GOOS)
+case $GOOS in
+linux)
+	__sdk_dir=$HOME/Android/Sdk
+	;;
+darwin)
+	__sdk_dir=$HOME/Library/Android/sdk
+	;;
+*)
+	echo "FATAL: unsupported operating system" 1>&2
+	exit 1
+	;;
+esac
+
+ANDROID_HOME=${ANDROID_HOME:-$__sdk_dir}
+
+__clitools_version=8512546
+__clitools_file=commandlinetools-linux-${__clitools_version}_latest.zip
+__clitools_sha256=2ccbda4302db862a28ada25aa7425d99dce9462046003c1714b059b5c47970d8
+
+printf "checking for curl... "
+command -v curl || {
+	echo "not found"
+	exit 1
+}
+printf "checking for shasum... "
+command -v shasum || {
+	echo "not found"
+	exit 1
+}
+printf "checking for unzip... "
+command -v unzip || {
+	echo "not found"
+	exit 1
+}
+
+set -x
+rm -rf $ANDROID_HOME/cmdline-tools/latest
+curl -fsSLO https://dl.google.com/android/repository/$__clitools_file
+echo "$__clitools_sha256  $__clitools_file" >__SHA256
+shasum --check __SHA256
+rm -f __SHA256
+unzip $__clitools_file
+rm $__clitools_file
+mkdir -p $ANDROID_HOME/cmdline-tools
+# See https://stackoverflow.com/a/61176718 to understand why
+# we need to reorganize the directories like this:
+mv cmdline-tools $ANDROID_HOME/cmdline-tools/latest