Make PrefixInformationOption struct constructor visible for testing.

PrefixInformationOption#build always gives the correct option length and
type, visible constructor is easiler to verify the incorrect parameters
in the test, for example, we will verify the code that parses the PIO
option in RA as a ND user option in netlink message.

Bug: 315069348
Test: TH
Change-Id: I6b88e6718cb873b15936c1f518d357c36c41ffde
1 file changed