Move DESCRIPTOR field to interface class and make it public

Previously the DESCRIPTOR field was a private field in the Stub class.

public interface IFoo extends IInterface {
    public static class Stub ... {
        private final static String DESCRIPTOR = ...;
    }
}

Now, it is

public interface IFoo extends IInterface {
    public static class Stub ... {
    }
    public final static String DESCRIPTOR = ...;
}

This changes makes java and c++ backends look similar; in c++ the
descriptor is already accessible via IFoo::descriptor.

Bug: 154650223
Test: aidl_unittests
Change-Id: I6953c92ad2f196a8c30468de8280ebb0af7b78b6
4 files changed