A build waiting to be installed arrives with a sound

Every notification this app posts was on one IMPORTANCE_LOW channel, so
all of them were silent -- including the row for a build that landed
while nobody was looking, which is the one that carries the Install
button and is the only thing here that is news rather than a report of
something somebody just pressed a button for.

A second channel rather than raising that one, because Android lets an
app only ever lower an existing channel's importance: raising "work" in
code would do nothing whatever on a phone that had already run this app,
and the whole fix would be somebody finding it in Android's settings.

The waiting row also has to undo the setOnlyAlertOnce every other row
wants. It replaces the running row it grew out of -- same tag, same id
-- and an update to a notification posted alert-once is silent however
loud its channel is, which is exactly the arrival nobody is watching
for.

Measured on the API 36 emulator: pressing Update on a test project and
going to the home screen posts the row on channel=ready, importance 3,
with mSound resolved and mIsInterruptive=true, and the shade files it
above its own "Silent" divider.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

README.md

@@ -135,7 +135,9 @@ A build that finishes while you are somewhere else leaves its notification
 up with an **Install** button on it, because Android will not let the app
 put the installer on screen from the background. Pressing it installs, and
 the notification goes away by itself — as the rest do when their work is
-over.
+over. That one arrives with a sound, and the progress ones do not: they
+are two channels, *Ready to install* and *Work in progress*, so either can
+be turned down on its own.
 
 ### Updating this server itself