cherry-pick 288b4ac9 into froyo

Killing the WifiWatchdogService thread from
WifiService can cause messages to be handled on
a dead thread. Quit the thread on the broadcast
instead.

A couple of more fixes:
- Do an asynchronous bring up of Wifi. This will
allow WifiWatchdogServiceThread to be immediately
brought up, instead of relying on an update.

- There is no need to listen on supplicant connection
in wifiwatchdog anymore. We kill the thread when
supplicant connection is no more.

Bug: 2546756
Change-Id: I15a188e031bc79856c55aabdd271287b0df0377d