WifiManager: make AsyncChannel static to avoid memory leak

WifiManager currently create new AsyncChannel for each WifiManager
instance, connect to WifiService everytime. Also, WifiService will
keep the AsyncChannel reference, and not release it. This will cause
memory leak. Make it static to solve the issue.

Change-Id: Icaae14a0592f1966ae9c611057305d6a9d7f6fc1