common: Annotate some methods in String

common: Annotate some methods in String

All variants of:

copyValueOf()
format()
intern()
toString()*
valueOf()

return a non-null value, so can be marked as such.

* Do not confound with Object.toString()! That one has no nullness
  guarantees.

Change-Id: Ia739dab7ab294e0c32156bd3b756f4241e97a4a1
Signed-off-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
Reviewed-on: https://git.eclipse.org/r/59163
Reviewed-by: Alexandre Montplaisir <alexmonthy@voxpopuli.im>
Tested-by: Alexandre Montplaisir <alexmonthy@voxpopuli.im>
Reviewed-by: Hudson CI