ice: decouple instantiation and initialization of ICE transport

Instantiation and initialization of ICE are done in a single
stage (in the class constructor).
To initialize the ICE instance, connection information must
first be gathered asynchronously.
However, when sharing ICE media instance between subcalls,
it's better to have a valid ICE instance as soon as possible
(even if not fully initialized) to proceed with the call
initialization process, then wait for ICE initialization to start
the call.
Thus, the ICE instantiation will be performed synchronously as
soon as the parent (main) call is created, then it will be
initialized asynchronously when the connection info are ready.

Gitlab: #619

Change-Id: I9c97516238f1a690603975ec968c8c6733155d4a