Bluetooth LE Advertising minor improvements

This patch adds some additional error checking for the advertising set
parameters, and some more comments.

Test: manual
Bug: 30622771
Change-Id: I87bd44f4179ef63694ad3ed656dc2acc52e40f1e
(cherry picked from commit f4ed33f5fa6ffa3bda6faff773a3fb90b16a760c)