Do not stop IpServer when recieve wifi ap disabling state.

This is used to fix the race when quickly OFF/ON wifi tethering.
When IpServer is started/stopped, there is callback update tethering
interface status. Before this change, IpServer is stopped when wifi
ap is disabling. Then the next startTethering may fail in wifi because
wifi is in disabling state.
Error pattern: WifiService: Tethering is already active.
No unitest for this CL but it fixed the CtsTetheringTest flakty rate
than around 30% to 0 for more than 100 runs.

Bug: 153925821
Test: atest CtsTetheringTest --iteration 100
Change-Id: I8b65f621abe20799a3a0d410ba1f06368746ee49