Retry networkAddInterface on EBUSY errorCode

If wifi and softAp shared with same interface and switching from wifi
to sotAp, there is race that tethering fail to set LCAL_NETID to softAp
interface because ConnectivityService do not release wifi netid from
that interface yet.

Bug: 158269544
Test: atest TetheringCoverageTests
      atest NetworkStackTests
Change-Id: I1dcf0a64190106820a93854b2eb53040341a97e6