Reuse proto fields in local LOI model

We set the value of `custom_tag` proto field to `customId` and `source` to `isPredefined` in the local LOI model. Ideally, we should reuse the proto fields in the local model as well.

We started working on this but later realized that it would require writing local db migrations as well since the db column name needs to be renamed as well. Ideally, we should club this change if there are any large breaking changes to prevent the extra effort of writing migrations for code health.

Split from https://github.com/google/ground-android/issues/2717