Backup Latin IME settings shared preferences.

Backup the shared preferences when they change.
diff --git a/AndroidManifest.xml b/AndroidManifest.xml
index 0ceac89..91a5537 100755
--- a/AndroidManifest.xml
+++ b/AndroidManifest.xml
@@ -5,7 +5,8 @@
     <uses-permission android:name="android.permission.READ_USER_DICTIONARY" />
     <uses-permission android:name="android.permission.WRITE_USER_DICTIONARY" />
 
-    <application android:label="@string/english_ime_name">
+    <application android:label="@string/english_ime_name"
+            android:backupAgent="LatinIMEBackupAgent">
 <!-- TODO: enable before donut release
         <supports-density android:density="120" />
         <supports-density android:density="160" />