service: Fix broken UUID advertising

The BTIF API expects a single 128-bit UUID in little-endian form to be
passed for its multi-adv service UUID field. The system service never
did this conversion so UUIDs were never properly advertised. This CL
fixes this.

Bug: 24247700
Change-Id: I541921da85bdabd907bfa9f75fbde7a3477ff593