floss: Add BleScanner topshim

Add topshim implementation for the GATT BLE scanner. This topshim
implementation uses cxx to shim the BleScannerInterface which uses lots
of C++ functionality.

Bug: 189497489
Tag: #floss
Test: emerge-zork floss
BYPASS_LONG_LINES_REASON: Bluetooth likes 120 lines

Change-Id: I4775379a34e117aaf09366947c331c7b79111420