gatt-service: change immediate alert and alert level UUID to 16Bit

Use 16Bit UUID format instead 128Bit, due to bugs related to convert
128Bits UUID. Also, the defines were renamed in order to avoid conflits
when import attr.h.
FIXUP: Shorter 16-bit UUID macros
gatt-service: Add Write Without Response property to Alert Level

Add Write Without Response property to Alert Level characteristic.
test: Add SIGINT/SIGTERM handler for gatt-service

This allows to cleanly shutdown and detect resource leaks.
test: Split service and characteristic interface registration
test: Support multiple services in gatt-service

Allow to customize Service and Characteristic parameters and register
multiple services.