WifiConnectivityManager: Refactor instantiation in ClientModeImpl

Create instance of WifiConnectivityManager in the constructor of
ClientModeImpl to avoid the null checks throughout the class. This
CL does not fix the ugly cyclic dependencies between these classes. That
is more involved and will be taken up later. This CL makes the
refactoring of WifiNetworkFactory from ClientModeImpl easier (next CL up
the stack).

Bug: 116218227
Test: Unit tests
Test: Device boots up & auto-connects to saved wifi networks.
Change-Id: Ib7b321482bf3f2eff9707e82a64da02513071e8e
5 files changed