[CLATJ#11] ClatCoordinator: config tun interface and bring up

Apply mtu and selected IPv4 address to tun interface.
Then, bring up the tun interface.

Bug: 212345928
Test: flash and boot
Run "atest ClatCoordinatorTest" in a followup commit.

Change-Id: I24a2a91e2b084be28820a7a63b265c5f56293334
1 file changed