build: use required platform in android-configure

The introduction of libuv 1.6.0 broke the android-configure script by
not specifying the correct platform. uv_os_homedir uses getpwuid_r
which was not made public until API level 21 on android.

The regression was introduced in a804026...b5cd2f0

PR-URL: #2501
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>

android-configure

@@ -6,7 +6,7 @@ $1/build/tools/make-standalone-toolchain.sh \
     --toolchain=arm-linux-androideabi-4.9 \
     --arch=arm \
     --install-dir=$TOOLCHAIN \
-    --platform=android-9
+    --platform=android-21
 export PATH=$TOOLCHAIN/bin:$PATH
 export AR=$TOOLCHAIN/bin/arm-linux-androideabi-ar
 export CC=$TOOLCHAIN/bin/arm-linux-androideabi-gcc