gn-build: Add bluetooth-service target

This patch makes the Bluetooth system service (service/) buildable using GN:

1. Added new BUILD.gn file for service/
2. Added conditional compilation for global config paths, with TODOs for
generalizing them later.
3. Added a shim for loading the Bluetooth library that calls hw_get_module on
Android and explicitly calls dlopen on OS_GENERIC.
4. Fixed compile warnings and errors.
5. Did some minor clean up in gatt_server.cpp for better readability.

Bug: 22124644
Change-Id: I3226537a3a5211a6762651a35707638df29956b0