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The user agent is no longer global and now depends on the linphone core
(kind of, there is still a global variable behind to hold its value).

This reverts commit 70955212.

It is also possible to check whether peer is under low bandwidth by looking into the linphone_call_get_remote_params()

- have the choice to keep same keys accross reINVITEs
- don't restart the stream for minor changes like removal of a recv-only codec.

The getReceiverInterarrivalJitter() function may be called before the
used audio codec is known in case of early media, so test that the
payload type is valid before using it.

Save playback/mic at config uninit