service/sdk: Change argument from proto to string.

Proto should be confined to the api/, switch it to take the path string.

BUG=None
TEST=run_pytest

Change-Id: I1a2db886f0e9c6503dd2d6c8277666c1caca490a
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/2340244
Tested-by: Alex Klein <saklein@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Michael Mortensen <mmortensen@google.com>
Commit-Queue: Alex Klein <saklein@chromium.org>
diff --git a/api/controller/sdk.py b/api/controller/sdk.py
index 3f345e0..33b477a 100644
--- a/api/controller/sdk.py
+++ b/api/controller/sdk.py
@@ -117,7 +117,7 @@
 @validate.validation_complete
 def UnmountPath(input_proto, _output_proto, _config):
   """Unmount a path"""
-  sdk.UnmountPath(input_proto.path)
+  sdk.UnmountPath(input_proto.path.path)
 
 
 @faux.all_empty