Adding accessible-hidden text to optional friend request label (Bug #1259768)

To alert screen reader that the message field is actually optional

Change-Id: I6356ba0da98faaa93f13de2225c3d2d1ddbba98f
Signed-off-by: Robert Lyon <robertl@catalyst.net.nz>
Merge "Adding accessible-hidden text to optional friend request label (Bug #1259768)"