file: Ensure nativeformats remains valid for lifetime of use.

It is possible for the nativeformats of a channel to change
throughout its lifetime. As a result a user of it needs to either
ensure the channel is locked when accessing the formats or keep
a reference to the nativeformats themselves.

This change fixes the file playback support so it keeps a
reference to the nativeformats when accessing things.

ASTERISK-25998 #close

Change-Id: Ie45b65475e1481ddf05b874ee48f63e39fff8915