Supply inflation hook is wrong

[gregdhill]

Describe the bug
We cannot call EscrowAnnuity::update_reward_per_block() because the escrow account also contains rewards already distributed to participants. We do the calculation rewardPerBlock = totalBalance / emissionPeriod where totalBalance should be the total amount for that emissionPeriod but is actually the balance of EscrowAnnuityAccount which also includes unclaimed tokens already distributed.

Expected behavior
We should calculate this value based on the inflation amount sent to the escrow account.

Additional context
We rectified the token distribution on Interlay and Kintsugi here:

• https://interlay.subsquare.io/democracy/referenda/87
• https://kintsugi.subsquare.io/democracy/referenda/132