Add get_session_token_if_present method

[bendk]

We're seeing lots of Android errors related to messing session tokens (https://bugzilla.mozilla.org/show_bug.cgi?id=2007418). However, it's not clear that Android really cares about the session token, see the bug.

Let's add a method that returns an optional string rather than throwing and switch Android to using this method. This matches the way they want to get session tokens, since they just catch any errors and return null anyway.

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[bendk]

After talking this through on Slack, it seems like the session token is expected to be present and it should be an error when it's not. I'm going to close this one and try to figure out how we can ensure that the session token is always there.