Floss: Add GATT interface for use by modules.

Bug: 224602924
Bug: 224603540
Bug: 224603198
Test: mma -j $(nproc)
Test: ./build.py
Test: system/gd/cert/run --clean --topshim
Tag: #floss
Change-Id: Ia25052be6b17e5cd322f79d0dec5d85ccc42bec7