Accept various formats for last_seen/last_timestamp attribute

[pnbruckner]

iCloud3 tracker entities have a last_located attribute. It seems to be a timestamp for the location attributes, and appears to always be a naive time string in HA's local time zone.

iCloud3 has been updated to create a last_timestamp attribute with a POSIX timestamp. So now it is supported.

Also enhance last_seen attribute processing to allow a datetime string format. Basically, these attributes can be one of the following:

• a naive Python datetime object which is assumed to be in HA's configured time zone
• an aware Python datetime object in any time zone
• anything homeassistant.util.dt.utc_from_timestamp will accept
• anything homeassistant.util.dt.parse_datetime will accept