Fixes a bug where (Signal) receiver_receipts could not be decoded with view_only FS.

[i-infra]

Use ViewAccountKey instead of AccountKey to use support check_receiver_receipt in view_only mode.

Motivation

Allows view-only FS accounts to process incoming Signal transactions.

In this PR

• We decode the AccountKey only as a ViewAccountKey when we just need to access the view_private_key and not the spend_private_key field.

Test Plan

I tested it manually, ie - with a bot.

Future Work

• Add test to ensure view-only decoding of receiver receipts is supported.
• Document how/why to limit access to spend_private_key.