Separate Struct.java into its own lib.

* Struct.java is removed from net-utils-device-common-bpf and
  net-utils-device-common-struct's source list
* Created a lib net-utils-device-common-struct-base for Struct.java
* Updated libs that require Struct.java to include
  net-utils-device-common-struct-base.
* Replaced net-utils-multicast-forwarding-structs with
  net-utils-device-common-struct in service-connectivity

Bug: 323503345

Test: build tethering module.
Change-Id: I3f60715c3d8043b7569db7612dee9d4fa1ccba59
12 files changed