libbinder: defork internal Android.bp

In order to fix divergence in libbinder Android.bp file, adding an
empty file group corresponding to an internal file group.

This is needed to easily manipulate where source files are included
in libbinder without having to land patches internally and externally.

Bug: 183654927
Test: N/A
Merged-In: Ie2a35186f2991fc4c3379c93c13b09f08e3a2bcf
Change-Id: I1c42a6a4f8ab9dc3d79a30e1b2824d1b0cbd5c4b