EthernetTetheringTest: add testTetherUdpV6

Add tethering IPv6 UDP test and fix a bug which may apply wrong
ethernet type to ethernet header builder.

Bug: 237369591
Test: atest EthernetTetheringTest
Change-Id: I0d619b6e338d082366d5b2c305047d8f2ebf630d
1 file changed