GPII-362: Updates the Ant build target to the Android 18 SDK.

@javihernandez can you take a look at this pull request? I'm not sure this is the appropriate approach, since project.properties is an autogenerated file. Running the latest Android Development Tools, however, I couldn't build the realtime framework without this fix.

Is this the correct way to do this, or should we be somehow modifying our prebuild.sh script or something else?