Add read only field to EntityInstance model

This flag will be used for passes imported from Wallet which cannot be
modified by the user.

Bug: 436175248

Change-Id: I1d160b5e55a907aa8d3afc2e6fa6383e768f7994
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6846204
Commit-Queue: Tobiasz Ciepliński <tobson@google.com>
Reviewed-by: Norge Vizcay <vizcay@google.com>
Reviewed-by: Florian Leimgruber <fleimgruber@google.com>
Cr-Commit-Position: refs/heads/main@{#1503295}