libbinder: hide Debug.h header

This is all an implementation detail for TextOutput which we want to
delete anyway. Since I'm working on binder sockets (and I don't want to
expand the usage of alog), I might add some other functions here, which
I don't want to expose from libbinder.

Bug: 148676615
Bug: 167966510
Test: N/A

Change-Id: Id5677c9ac195dac046becc5d36b02bbdd1f7c339