common: Annotate ByteBuffer.array()

common: Annotate ByteBuffer.array()

to make it return a "byte @NonNull []", which means that
the array reference cannot be null.

It returns the buffer's internal array, and the implementation
first checks if it is null, and throws a
UnsupportedOperationException if it is.

Change-Id: I91d8a90d11dbe30ba0895103fadba78ed4b35ecf
Signed-off-by: Alexandre Montplaisir <alexmonthy@efficios.com>
Reviewed-on: https://git.eclipse.org/r/70336
Reviewed-by: Hudson CI
Reviewed-by: Matthew Khouzam <matthew.khouzam@ericsson.com>