Align pub_date_tmsp with iOS

[wzieba]

First raised: #63 (comment)

Description

There are two differences between iOS and Android when it comes to declaring date of publication:

1. Android adds this field as metadata.pub_date_tmsp while iOS as metadata.pub_date
2. Android uses seconds there while iOS - milliseconds

We probably should align Android to match iOS behavior.

[github-actions]

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