core: Add API to retrieve mtu size

Adding an API to libnetutils to retrieve mtu size of an interface
Made few utility functions from ifc_utils public

Change-Id: Iddec2dcd9f241b146d7f362e2df0724cf740d1f2
(cherry picked from commit 108cded7d96cc347f36b5b78c404c5ebccce13d7)
libnetutils: add ifc_set_mtu

Change-Id: I3031e9ee38583648350f2c46baa7a9a714b9ea1e
Do not add default route during ifc_configure

Avoid duplicate default routes.
Remove adding of default routes during interface configuration as
route add/remove operation is managed by ConnectivityService via netd.
(cherry picked from commit 4a48635c72d91cf8decb7653948fe0c76de46098)

Change-Id: I982b95a44bf91d3850ca7752e307c1b1b6a66ecf