Fix duplicated User values when availability changes

When the availability has updated, the updated user was
appended, but the prvious value was never removed, causing
the list to continue growing.

Test: atest IntentResolver-tests-unit
Test: onHandleAvailabilityChange_userStateMaintained
Bug: 324073704
Change-Id: I3c51ea9e1a2cf0de9c064b08e7c9f21c4c0f7af1