Add support for Android 2.3+

There have been plenty of ppl involved in creating this version.
I don't wanna mention names as I'm sure I'd forget someone so I
just tell where help has been done:
- The partial android versions done by various ppl
- Testing on different android devices
- reviewing code (especially the in core changes)
- testing controls
- reviewing texts

A big thank you to everyone helping this to be completed!

build/android/build.xml

@@ -0,0 +1,16 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<project name="Minetest" default="help">
+	<property file="local.properties" />
+	<property file="ant.properties" />
+	<property environment="env" />
+	<condition property="sdk.dir" value="${env.ANDROID_HOME}">
+		<isset property="env.ANDROID_HOME" />
+	</condition>
+	<loadproperties srcFile="project.properties" />
+	<fail
+		message="sdk.dir is missing. Make sure to generate local.properties using 'android update project' or to inject it through the ANDROID_HOME environment variable."
+		unless="sdk.dir"
+	/>
+	<import file="custom_rules.xml" optional="true" />
+	<import file="${sdk.dir}/tools/ant/build.xml" />
+</project>