[XFRM_MSG_GETSA] Fix the class names and code locations

- Change class names to start with Xfrm or StructXfrm for consistency
  with other messages and structs
- Create xfrm sub-folder for xfrm messages
- Add "finals" to input parameters

Bug: 308011229
Test: atest NetworkStaticLibTests:com.android.net.moduletests.util.netlink
      (new tests added)
Change-Id: Id6d34b4549bbc1890b3f4dd9f1638977f66e3236