IWindowSession.Default is not available

<Iface>.Default class was automatically generated, but a change in the
AIDL compiler is scheduled to generate the class only when the AIDL file
is compiled with '-version' option, because the default class is only
useful for the versioned AIDL case.

Stop using IWindowSession.Default since it will be removed. Instead
implement IWindowSession directly.

Test: build
Change-Id: I4551ddcbd794d0d99dc0952bbd6414a2112a5ea2