Add storage library (and demo app)

and use for periodic files backup
diff --git a/permissions_com.stevesoltys.seedvault.xml b/permissions_com.stevesoltys.seedvault.xml
index da36b24..62051bc 100644
--- a/permissions_com.stevesoltys.seedvault.xml
+++ b/permissions_com.stevesoltys.seedvault.xml
@@ -5,5 +5,6 @@
         <permission name="android.permission.MANAGE_USB"/>
         <permission name="android.permission.INSTALL_PACKAGES"/>
         <permission name="android.permission.WRITE_SECURE_SETTINGS"/>
+        <permission name="android.permission.MANAGE_EXTERNAL_STORAGE"/>
     </privapp-permissions>
 </permissions>
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