ipn/ipnlocal: make GetExt work earlier, before extension init

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>
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