tmf: Null-annotate state system API classes

tmf: Null-annotate state system API classes

The main culprits here are the createStateProvider() method, that need
to pass a @NonNull trace, but simply take the result of getTrace() which
is already marked @Nullable. Simply wrapping them in checkNotNull() calls
feels lazy, but it's still better than the current situation.

Change-Id: Ic1ceb149cbb06e6f91a3351ef757231907a67ae2
Signed-off-by: Alexandre Montplaisir <alexmonthy@voxpopuli.im>
Reviewed-on: https://git.eclipse.org/r/38308
Reviewed-by: Hudson CI
Reviewed-by: Matthew Khouzam <matthew.khouzam@ericsson.com>