When the 'when' timer of the notification is disabled
`DefaultMediaNotificationProvider` may set `C.TIME_UNSET`
as the time. Users reported problems on some devices with
this and the docs ask for an event time that probably
shouldn't be a negative number.
This change sets `0L` instead of `C.TIME_UNSET` when the
timer is disabled.
Issue: androidx/media#903
#minor-release
PiperOrigin-RevId: 594451074
(cherry picked from commit 426bc940905237accf902f42d63b3777c11ff72e)
d1b882ae56