Make libstagefright_aacenc depend on libbinder

This library includes libstagefright headers, which in turn
include libbinder headers.  Unfortunately, introducing a
dependency on libstagefright directly introduces a circular dependency.

Bug: 27804373
Change-Id: Id1e65f99b63275ffdccb05638412f43825f8ec1c
Test: Compiles with accompanying changes.