Add workaround for b/124210145

Problem description:
Metalava emits signature of a method under a class, even when the method
is not defined in the class when the method is inherited from a hidden
ancestor and the method is part of the public interface that the class
is expected to implement.

To be specific, inside the api signature, MmbmsDownloadServiceBase has
asBinder() and onTransact(). The methods are not defined in the class
but inherited from the auto-generated hidden class
IMbmsDownloadService.Stub. However, since the methods are also declared
in the public ancestors of the class, e.g., Binder and IInterface,
the methods are force-included in the class. Omitting the methods will
cause problem when building the stub version of the class.

This inclusion of the inherited method is breaking
SystemApiAnnotationTest. That's because the test ensures that a symbol
listed in the API signature file for the System API is actually
annotated with @SystemApi.

Solution:
To workaround the issue, actually implement the auto-generated methods
inside the class and annotate them.

Bug: 124210145
Test: atest CtsSystemApiAnnotationTestCases
Change-Id: I6760f6e4068239361c495ae7c0de3e25f91d38e1