Update type in RoutingAttribute

The length in RoutingAttribute use getshort() from ByteBuffer.
Theoretically, it should be stored as a short. Also, similar
structure alignment is implemented in alignedLengthOf() in
NetlinkConstant. Update to use the same function.

Bug: 145275899
Test: atest NetworkStackTests NetworkStackNextTests
Change-Id: Ibabb15ee0a96b4c0331cc080b578c625e10d4b37