Make RtNetlinkLinkMessage class immutable after creation

This CL marks all the fields of RtNetlinkLinkMessage class final. That
means once the instance is created, the instance is immutable.

Also, the CL adds a static create method that verifies the arguments,
and updates the nlmsg_len field of the header before creation.

Bug: 364501824
Test: atest NetworkStaticLibTests

Change-Id: I08af4a6e68df9aee8ae76d953fac5d81f0b02c84
2 files changed