Set legacy MTU field for backward compatibility

For IWLAN, MTU is not negotiated as part of the RA. The interface
MTU is still necessary here to ensure full backward compatibility.
To workaround this, we set legacy mtu field with the larger value
of mtu v4 and mtu v6.

Fix: 153470573
Test: FrameworkTelephonyTests
Change-Id: I0671618817a295cc4e9c75df17501de338dbe155