Tell the user to unlock/confirm backup & restore operations

People are being confused about the silent failure of backup/restore
when they didn't know they had to unlock the device & confirm the
backup/restore operation, so now adb prints a brief reminder.

Change-Id: I1b32913f0ad0cf6e30bf235dc975b9e983b533fd
diff --git a/adb/commandline.c b/adb/commandline.c
index 31ba3ac..ffce883 100644
--- a/adb/commandline.c
+++ b/adb/commandline.c
@@ -648,6 +648,7 @@
         return -1;
     }
 
+    printf("Now unlock your device and confirm the backup operation.\n");
     copy_to_file(fd, outFd);
 
     adb_close(fd);
@@ -675,6 +676,7 @@
         return -1;
     }
 
+    printf("Now unlock your device and confirm the restore operation.\n");
     copy_to_file(tarFd, fd);
 
     adb_close(fd);