Use BpfDump.toBase64EncodedString to dump raw map

Move base64 related bpf map function and constant to f/l/n.
This is preparation for testing BPF map in ClatCoordinator.

Test: atest EthernetTetheringTest
Change-Id: Ieb1e0a0f128c9aa0d7c9f043e90eecf3c61252f5
2 files changed