Make BaseNetworkObserver available to core code

Currently BaseNetworkObserver is in frameworks/base/services, but
there is code in frameworks/base/core that could use it. This
code typically extends INetworkManagementEventObserver.stub
because BaseNetworkObserver is not available.

Move BaseNetworkObserver to frameworks/base/core without changing
its package name, and use it to simplify two callers. The third
caller, Tethering, is much larger, and I'm not sure it's
appropriate to change it.

Bug: 10232006
Change-Id: Ifc0f2e619e3424e27e35730c048a1cc523df345e