Set the default Android platform to 21

Also explain that the "zip" command is required to build AAR archives
and that NDK_PLATFORM can be used to override the target platform.

Fixes #1440

dist-build/android-aar.sh

@@ -9,6 +9,13 @@ SODIUM_VERSION="1.0.20.0"
 NDK_VERSION=$(grep "Pkg.Revision = " <"${ANDROID_NDK_HOME}/source.properties" | cut -f 2 -d '=' | cut -f 2 -d' ' | cut -f 1 -d'.')
 DEST_PATH=$(mktemp -d)
 
+if which zip >/dev/null; then
+  echo "The 'zip' command is installed."
+else
+  echo "The 'zip' command is not installed. Please install it before running this script."
+  exit 1
+fi
+
 cd "$(dirname "$0")/../" || exit
 
 make_abi_json() {

dist-build/android-build.sh

@@ -1,7 +1,8 @@
 #! /bin/sh
 
 if [ -z "$NDK_PLATFORM" ]; then
-  export NDK_PLATFORM="android-19"
+  export NDK_PLATFORM="android-21"
+  echo "Compiling for default platform: [${NDK_PLATFORM}] - That can be changed by setting an NDK_PLATFORM environment variable."
 fi
 export NDK_PLATFORM_COMPAT="${NDK_PLATFORM_COMPAT:-${NDK_PLATFORM}}"
 export NDK_API_VERSION="$(echo "$NDK_PLATFORM" | sed 's/^android-//')"