screen-saver-manager: Blank on power button release

This uses phoc!431 (merged) to blank on power button release. Otherwise we can unblank right away if the power button is pressed for longer periods of time (see phoc!429 (closed)).

This code should run against older phoc (showing the old behavior) so we don't need a new hard dependency on newer phoc yet (to ease bisecting, etc). I want to retest that though, hence draft (also since we want to wait until the phoc protocol change landed).

Note that this needs the phoc side to work (phoc!431 (merged))

I'll post the code that leverages power button long press separately. (See !1224 (merged))

Closes: #911 (closed)