Add an XPath Ant task to be used in the android_rules.xml file.

This task is designed to run XPath expression targetting Android XML files.
It will handle the Android namespace with the 'android' prefix, and store
the result of the XPath into a property.

Change-Id: I9094f5661d5e1ef86553ee1c54bdeca62366e0cd
classname: XPathTask
classpath: anttasks.jar, sdklib.jar

Usage:
<xpathtask input="path/of/file/to/read" output="name.of.property.to.write"
    expression="/xpath/expression/to/evaluate" />