wifi: Update AbstractWifiService to be concrete.

This patch removes the abstract qualifier from AbstractWifiService and
change its name to BaseWifiService for consistency.

This checks at compile time that all base methods defined in
IWifiManager.aidl are stubbed in AbstractWifiService. Previously since
AbstractWifiService was an abstract class, any new method added to
IWifiManager.aidl would not force an update to AbstractWifiService.

Bug: 112338174
Test: Checked that removing a stub from BaseWifiService causes an
error at compilation.

Change-Id: I3f71aae51adfe5775bec25e1133876b975cf0515