[smart-select:] build with -fvisibility=hidden, to cut .so code size.

The .so produced by mmma -j16 external/libtextclassifier,
out/target/product/bullhead/obj_arm/lib/libtextclassifier.so, goes down in size
361.7K -> 273.0K

Test: zilka@ manually tested the 32/64 bit libs on a real phone.
Change-Id: I5d373b9f8e75bb4120cda1ed15196fdf4c67b8d9