Simplify IWeaver mocking code.

Implementations of HIDL interfaces are intended to extend
from the Stub class, not the core interface class.

Test: m FrameworksServicesTests
Test: frameworks/base/services/tests/runtests.py -e package "com.android.server.locksettings"
OK (454 tests)

Bug: 112334405

Change-Id: Ib060560fd74f0092544096210f52e1f7fcba6c06