refactor(sdk): Split power_levels in latest_event.

This patch splits `power_levels: &Option<(&UserId, RoomPowerLevels)>`
into 2 variables: `own_user_id: Option<&UserId>` and `power_levels:
Option<&RoomPowerLevels>`. The idea is to be able to get the
`own_user_id` even if the power levels are `None`.

Author: Hywan