Replace getCreationTime clock with elapsedRealtime

It is only used by PreperationCoordinator, with an expected time
difference of under 500ms, so the difference should have no effect
there. It allows us to use the creation Time when determining how old an
OTP notification actually is, and compare that with the wall clock
getWhen. Other uses only rely on uptimeMillis increasing monotonically,
which elapsedRealtime does as well.

Bug: 401173968
Test: atest DeviceProvisionedCoordinatorTest NotificationEntryTest
VisualStabilityCoordinatorTest
Flag: android.app.notif_entry_creation_time_use_elapsed_realtime

Change-Id: Id351090b0b4d6538da1dd8de305d43f8db7374ba