WifiNative: Handle link state changed in wifi thread

The network observer callback is invoked by connectivity service from
the main system thread. Post the processing to the wifi thread to avoid
blocking the system.

Bug: 117438653
Test: Unit tests
Test: Toggled wifi/softap on/off several times.
Test: Will send for regression tests
Change-Id: I6f48e68693a2da40a9332b43be07d056f516ddad
4 files changed