Only stop/start clatd if necessary.

Previously we would restart clatd on every LinkProperties
change, which now happens every time we switch radio technology
(e.g., LTE to HSPA). We also would not stop it if the link got
an IPv4 address.

Bug: 15024258
Bug: 17186694
Bug: 17569702
Change-Id: I65cfcd5e7acec8ea1a12392a59dabd668c58490f