identity: Clarify validity period for auth keys.

The AIDL said "one year in the future (365 days)" which is not a
constant amount of time given that leap years and leap seconds may
exist. Change this to be a constant amount of seconds instead.

Bug: 271948315
Test: atest VtsHalIdentityTargetTest
Change-Id: I324a809900feb898d2343e02066f2fb237a46e86