Load /metadata/password_slots/slot_map lazily.

This fixes a 6ms regression in SystemServerTiming_MakeLockSettingsServiceReady, since the slot map is only needed when adding or removing a PIN.

Bug: 128334055
Test: atest frameworks/base/services/tests/servicestests/src/com/android/server/locksettings
Test: atest PasswordSlotManagerTests
Test: atest google/perf/boottime/boottime-test
Test: PIN unlocks device after booting into GSI
Change-Id: Ib2e0c05575d07496792188716b962ca622df28e6