[Status Network Contracts] Total MP accrual starts too soon

[0x-r4bbit]

Context: https://github.com/Cyfrin/audit-2025-12-statusl2/issues/21

TLDR:

• lastMPUpdatedTime is set at initialization
• After some time (say 1 year) first user stakes
• lastMPUpdatedTime is not updated because newTotalMPAccrued will be 0 (before first stake there's no MP)
• More time passes (say another year), some users calls updateVault() (which updates totalMPAccrued()
• totalMPAccrued() now yields MP amount for 2 years (since initialization) instead of 1 year (since first stake)