Adding support for DB downgrade

Adding a schema file for handling DB downgrade. This schema file is part of
the backup/restore set, and hence is available on a device with lower app version.

Bug: 37257575
Change-Id: I69c8ef5f28d5209be6e6679412c7459d4eeda5d0
diff --git a/res/xml/backupscheme.xml b/res/xml/backupscheme.xml
index 7e833a0..299e92e 100644
--- a/res/xml/backupscheme.xml
+++ b/res/xml/backupscheme.xml
@@ -3,5 +3,6 @@
 
     <include domain="database" path="launcher.db" />
     <include domain="sharedpref" path="com.android.launcher3.prefs.xml" />
+    <include domain="file" path="downgrade_schema.json" />
 
 </full-backup-content>
\ No newline at end of file