118395019: Hide constants and methods in StatsLogInternal

This CL updates the code generator which creates the
hidden StatsLogInternal class to explicitly hide
the generated constants and write methods as well.

The intent of this class was for everything to be hidden,
but it turns out that public methods and fields in
hidden classes which are extended by a public class also
ends up in the SDK, even though they don't appear in the
signature file. StringBuilder#setLength(int) is an
example of this.

Bug: 118395019
Test: make sdk
Change-Id: I97e510e8155ee50ade653f6abeb5479c7ca9029d