bin: Post a LATENCY message with async-handling=TRUE if the PLAYING state is reached

bin: Post a LATENCY message with async-handling=TRUE if the PLAYING state is reached

Otherwise each bin might have a different latency in the end, causing
synchronization problems.

The bin will still first handle latency internally as before, but gives the
overall pipeline the opportunity to update the latency of the whole pipeline
afterwards.

https://bugzilla.gnome.org/show_bug.cgi?id=759125