Specify the latest android platform to eliminate extraneous build warnings.

The interface in the error message below exists only after API Level 18.

[jar2xml] -jar "/svn/monodroid/monodroid/out/lib/mandroid/jar2xml.jar" --out="obj/platform-4/generated-api.xml" --jar="/home/atsushi/android-sdk-linux/extras/android/support/v4/android-support-v4.jar" --ref="/home/atsushi/android-sdk-linux/platforms/android-17/android.jar" --droiddocpath="/home/atsushi/android-sdk-linux/docs/reference"
java.lang.NoClassDefFoundError: android/view/ViewTreeObserver$OnWindowAttachListener
	at java.lang.ClassLoader.defineClass1(Native Method)
	at java.lang.ClassLoader.defineClass(ClassLoader.java:800)
	at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:142)
	at java.net.URLClassLoader.defineClass(URLClassLoader.java:449)
	at java.net.URLClassLoader.access$100(URLClassLoader.java:71)
	at java.net.URLClassLoader$1.run(URLClassLoader.java:361)
	at java.net.URLClassLoader$1.run(URLClassLoader.java:355)
	at java.security.AccessController.doPrivileged(Native Method)
	at java.net.URLClassLoader.findClass(URLClassLoader.java:354)
	at java.lang.ClassLoader.loadClass(ClassLoader.java:425)
	at java.lang.ClassLoader.loadClass(ClassLoader.java:358)
	at jar2xml.JavaArchive.getPackages(JavaArchive.java:86)
	at jar2xml.JavaArchive.getPackages(JavaArchive.java:64)
	at jar2xml.Start.main(Start.java:126)