[FIX] mail: remove behaviours dependent on connectionState

Before this commit, there were some usages of
RTCPeerConnection.connectionState which is not available on some
browsers, including Firefox.

This commit fixes this issue and make it so that we only use
RTCPeerConnection.iceConnectionState instead.

closes odoo/odoo#77890

Signed-off-by: Sébastien Theys (seb) <seb@odoo.com>