Separate FileUtils implementations for Android and desktop builds.

The method used by Commons IO to get the available disk space fails on
Android devices that lack a df binary - use the Android API instead.

briar-api/src/net/sf/briar/api/os/FileUtils.java

@@ -0,0 +1,9 @@
+package net.sf.briar.api.os;
+
+import java.io.File;
+import java.io.IOException;
+
+public interface FileUtils {
+
+	long getFreeSpace(File f) throws IOException;
+}