WifiManager: Retrieve IWifiManager service lazily

Wifi services will be initialized a little later in the bootup sequence
because it is now a separate APK. So, retrieve wifi services lazily
(on the first method call on the object).
This will alleviate the need to modify all the clients which instantiate
WifiManager very early in the bootup sequence (Sysui, some vendor apps,
etc).

Bug: 137374831
Test: Device boots up & connects to wifi networks.
Change-Id: I8e1f0eb3a5a8fdb79b088447fbb9f97413f64483
Merged-In: I8e1f0eb3a5a8fdb79b088447fbb9f97413f64483