wgengine{,/monitor}: restore Engine.LinkChange, add Mon.InjectEvent

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The Engine.LinkChange method was recently removed in
e3df29d488 while misremembering how
Android's link state mechanism worked.

Rather than do some last minute rearchitecting of link state on
Android before Tailscale 1.6, restore the old Engine.LinkChange hook
for now so the Android client doesn't need any changes. But change how
it's implemented to instead inject an event into the link monitor.

Fixes #1427

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

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