WifiManager: Add new API's for backup/restore

Expose new API's to retrieve data to be backed up for Wifi
configurations and to restore configurations from the data.
BackupSettingsAgent will invoke these API's to backup/restore wifi
settings.

BUG: 28967335
Change-Id: I64e2f37a29d538d976bcc53dd2165653073b5862
(cherry picked from commit 7bc0adbced)

wifi/java/android/net/wifi/IWifiManager.aidl

@@ -169,5 +169,9 @@ interface IWifiManager
     void factoryReset();
 
     Network getCurrentNetwork();
+
+    byte[] retrieveBackupData();
+
+    void restoreBackupData(in byte[] data);
 }
 

wifi/java/android/net/wifi/WifiManager.java

@@ -2737,4 +2737,28 @@ public class WifiManager {
             throw e.rethrowFromSystemServer();
         }
     }
+
+    /**
+     * Retrieve the data to be backed to save the current state.
+     * @hide
+     */
+    public byte[] retrieveBackupData() {
+        try {
+            return mService.retrieveBackupData();
+        } catch (RemoteException e) {
+            throw e.rethrowFromSystemServer();
+        }
+    }
+
+    /**
+     * Restore state from the backed up data.
+     * @hide
+     */
+    public void restoreBackupData(byte[] data) {
+        try {
+            mService.restoreBackupData(data);
+        } catch (RemoteException e) {
+            throw e.rethrowFromSystemServer();
+        }
+    }
 }