android: Build and link with zlib

Previously, we linked with zlib shipped with NDK, but it seems this is
not part of NDK API, and thus could break our app.

android/zlib-config

@@ -0,0 +1,22 @@
+#!/bin/sh -e
+
+if [ -z "$ANDROID_HOME" ]; then
+    echo 'No $ANDROID_HOME specified.'
+    exit 1
+fi
+PREFIX=$ANDROID_HOME/usr/local
+TOOLCHAIN=$ANDROID_HOME/toolchain
+PATH=$TOOLCHAIN/bin:$PATH
+
+HOST=arm-linux-androideabi
+
+CC=$HOST-gcc \
+AR=$HOST-ar \
+LD=$HOST-ld \
+RANLIB=$HOST-ranlib \
+STRIP=$HOST-strip \
+./configure \
+    --prefix=$PREFIX \
+    --libdir=$PREFIX/lib \
+    --includedir=$PREFIX/include \
+    --static