tmf: Add a method to query an ongoing state's start time

tmf: Add a method to query an ongoing state's start time

We can already query a state system's current state value for
a given attribute. It's trivial to expose a method to also query
the start time of that state.

Change-Id: I4b2610709bb0af33b612514007787b20f941f490
Signed-off-by: Alexandre Montplaisir <alexmonthy@voxpopuli.im>
Reviewed-on: https://git.eclipse.org/r/10757
Tested-by: Hudson CI
Reviewed-by: Bernd Hufmann <bhufmann@gmail.com>
IP-Clean: Bernd Hufmann <bhufmann@gmail.com>