Scope of System.setProperty
Android Activities of the same application, if you don't specify otherwise in the Manifest.xml, will run in a default Process created for the application when it starts.
Each process runs in its own JVM. So the System.setProperty(), based on @mmccomb 's answer, will reach all Activities in the same Application except if you, on pupose, declare Activities to run in different Processes.
See http://developer.android.com/guide/components/processes-and-threads.html