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

Conflicts:
	wifi/java/android/net/wifi/WifiManager.java