floss: adv: Remove unnecessary pub(crate)

After https://r.android.com/2968363 most of the helper functions in
AdvertiseManager don't need to be pub to the crate. Removing them.

Bug: 242083290
Tag: #floss
Test: mmm packages/modules/Bluetooth
Test: USE=floss_upstream emerge-nami floss
Flag: EXEMPT, Floss-only changes
Change-Id: I0dced36844ca53e26225785816d902c47e2959d1