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

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

MOBILE/android/ensure

@@ -0,0 +1,38 @@
+#!/bin/bash
+
+set -euo pipefail
+
+__install_extra="build-tools;32.0.0 platforms;android-31"
+
+__ndk_version="23.1.7779620"
+
+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}
+if [[ ! -d $ANDROID_HOME ]]; then
+	echo "FATAL: expected to find android SDK at $ANDROID_HOME, but found nothing" 1>&2
+	echo "HINT: run ./MOBILE/android/setup to (re)install the SDK" 1>&2
+	exit 1
+fi
+
+__sdkmanager=$ANDROID_HOME/cmdline-tools/latest/bin/sdkmanager
+if [[ ! -x $__sdkmanager ]]; then
+	echo "FATAL: expected to find sdkmanager at $__sdkmanager, but found nothing" 1>&2
+	echo "HINT: run ./MOBILE/android/setup to (re)install the SDK" 1>&2
+	exit 1
+fi
+
+set -x
+echo "Yes" | $__sdkmanager --install $__install_extra "ndk;$__ndk_version"