tmf: Annotate methods in ITmfEventField

tmf: Annotate methods in ITmfEventField

Some methods in ITmfEventField can be annotated @NonNull to make
their usage easier.

getValue() and getField() should eventually be marked @Nullable,
but this will be a much larger undertaking, as most analysis
modules currently just get fields and values without even
checking them...

Change-Id: I564c6d067a62239cde5c67350ea709315bfe89f4
Signed-off-by: Alexandre Montplaisir <alexmonthy@voxpopuli.im>
Reviewed-on: https://git.eclipse.org/r/51212
Reviewed-by: Hudson CI
Reviewed-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
Tested-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
Reviewed-by: Patrick Tasse <patrick.tasse@gmail.com>