title: Android "remembers" shared preferences after complete app uninstall
body: <p>In my Android app I use a set of randomly generated file names to store some data. In order to ensure that the same set of file names are generated at app restart - and are yet different for each app installation - I start off the process by picking the names as substrings from a long string of random alphanumeric characters which I generate when the app is installed. This latter string is stored in Shared Prefereneces.  </p>
  
  <p>As I am testing out the app I have run into a rather peculiar issue.  From time-to-time I make major changes so I fully uninstall the app - and even Force Close it + clear all its data.  At that point I would expect that the device would have no "prior knowledge" of the app if it is reinstalled.  And yet I find that the Shared Preferences string is somehow "remembered".  This causes havoc if in the interim I have changed how the file name substrings are picked up from the stored shared preferences string.</p>
  
  <p>How can I ensure that the app has "zero memory" of a previously installed version that has subsequently been uninstalled?</p>
  
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  <p>One solution I have used in the past is to instruct Android not to do any backups via the manifest file, <code>android:allowBackup = "false"</code>. However, I have backed away from that idea since - unless I am mistaken - it effectively means that I am stopping the user from porting their app over to a new device when they decide to change handsets.</p>