Reland "[Nearby Presence] Flag BleV2Medium creation in platform.cc"

This is a reland of commit 7885f9bf5626c4e57e9f6eeec19a89bac33c2178
since the crash fix to handle the nullptr is landing in crrev/c/5556026

Original change's description:
> [Nearby Presence] Flag BleV2Medium creation in platform.cc
>
> Adds a flag to return the BleV2Medium class in platform.cc, which is
> required in order to prevent BleV2Medium from being returned while
> we are still in development. Other classes do not need this check
> because they rely on flags on the other side of the mojo pipe which
> determine whether or not the passed in Mojo remotes are bound or not,
> however, since BleV2Medium uses the same BluetoothAdapter remote, we
> cannot depend on this as a way to flag it's creation, and need to
> additionally check the flag.
>
> Tested: Manually verified on DUT
> Bug: b/333834234
> Change-Id: Ic8c4511d397417d59875882154065f51cc80ed55
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5551113
> Reviewed-by: Jack Shira <jackshira@google.com>
> Commit-Queue: Juliet Lévesque <julietlevesque@google.com>
> Cr-Commit-Position: refs/heads/main@{#1303493}

Bug: b/333834234
Change-Id: Id50eef1e796b05591af7f2aaaa7276275d5a36ec
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5553023
Commit-Queue: Juliet Lévesque <julietlevesque@google.com>
Reviewed-by: Jack Shira <jackshira@google.com>
Cr-Commit-Position: refs/heads/main@{#1304200}

• chrome/services/sharing/nearby/platform.cc[diff]