WiFi: Define a base abstract class for IWifiManager

This patch creates a base abstract class that implements
IWifiManager.aidl with stub methods that throw runtime exceptions.

Actual implementations of IWifiManager can extend this class instead of
directly extending the autogenerated IWifiManager.Stub.

This allows adding new methods or modifying existing methods in
IWifiManager without requiring a synchronized change on all concrete
implementations.

Bug: 112338174
Test: Compiled, flashed marlin, booted. WiFi works.
Change-Id: I316c999d6c076f4e6ea1354bd558512915d551ed