Floss: Add server_read_phy topshim in GattServer

This adds server_read_phy and its callback to topshim with
CXX since we can't pass base::Callback with bindgen

Bug: 193685149
Tag: #floss
Test: emerge floss
Change-Id: Ic4924f8f465bf9c76d7110eb5d4f869e3e14b002