[Nearby Connections] MVP for StartAdvertising on BLE v2

Implements the MVP for StartAdvertising on BLE v2. This function works
properly, and was tested with Fast, Legacy, and Extended advertisements.

Test: Manual testing and unit tests.
Bug: b/320554697
Bug: b/330759317
Change-Id: I94f473a6f4ed95ee312546ac42f8f33e06ecc8f4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5347437


Reviewed-by: Ryan Hansberry <hansberry@chromium.org>
Commit-Queue: Daniel Classon <dclasson@google.com>
Reviewed-by: Jack Shira <jackshira@google.com>
Cr-Commit-Position: refs/heads/main@{#1277169}