libbinder: hide BufferedTextOutput.h

Nothing uses this now, and the general feeling is that we should
deprecate TextOutput at some point in favor of liblog (potentially to
save a small amount of memory from static TextOutput objects). So,
hiding this header.

Bug: N/A
Test: builds
Change-Id: I4f1d8a1aa8a5db4e602da6499050881ab4aaeca2