Taildrop wasn't working on iOS since #15971 because GetExt didn't work
until after init, but that PR moved Init until after Start.
This makes GetExt work before LocalBackend.Start (ExtensionHost.Init).
Updates #15812
Change-Id: I6e87257cd97a20f86083a746d39df223e5b6791b
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>