Add getExtension to android.os.IBinder interface

Provide a mechanism to extend and customize a binder interface without
having to modify the original (base) interface.
See the full motivation here: aosp/1099664

Bug: 147834064
Test: atest FrameworksCoreTests:BinderTest FrameworksCoreTests:BinderProxyTest
Change-Id: Idd3595b1302cf7ac0590d064e795ba0b75811af6