Explicitly enable backup by default in SDK builds

This change parallels a frameworks/base change that makes the default
backup state at boot "enabled, with LocalTransport as the currently
active transport."  Device vendors who wish to ship the Google backup
transport need to explicitly disable backup by default (it is enabled
when the user agrees to the opt-in disclosure).

Change-Id: I6b0c5c059ef58effd5c121107ea3a5edd9e936cc

Author: Christopher Tate

sdk_overlay/frameworks/base/packages/SettingsProvider/res/values/defaults.xml

@@ -17,5 +17,8 @@
  */
 -->
 <resources>
+    <!-- The SDK builds do not talk to a remote backend, and enabling by
+         default makes for the best developer experience. -->
+    <bool name="def_backup_enabled">true</bool>
     <string name="def_backup_transport">android/com.android.internal.backup.LocalTransport</string>
 </resources>