android: say nothing until the update is downloaded, and only fetch on wifi

Both corrections to what I built, and the second changes the first.

NAG ONLY WHEN READY. The banner is now gated on the bytes being on disk. I had it
appearing as soon as a build was FOUND, with Install downloading on demand — which
turns one tap into an unplanned download, and is exactly the surprise the wifi gate
was meant to avoid. Off wifi the app now stays quiet and picks it up later.

ONLY ON WIFI, and both halves of that. `isActiveNetworkMetered` alone would download
over an unmetered cellular plan, which is not what "on wifi" means. TRANSPORT_WIFI
alone would download over a tethered hotspot, which is mobile data wearing a
different hat and the precise bill this avoids. It now requires both.

Found while making the first change: gating the nag on `ready` broke the nag. The
background path returns early once a build is fetched, so `nagDismissed` would never
be cleared again and a single "Later" would have silenced the update permanently —
the exact "lost" this whole path exists to prevent. Coming forward with a fetched
build now clears the dismissal instead of returning.

Also: a build found off wifi retries its FETCH on the next foreground rather than
waiting out the six-hour check interval. Found on the train, downloaded at home.

The banner loses its two-state text with the change, and BoardUpdate loses `ready` —
it is implied now. It stays visible while installing, deliberately: that is the one
moment it has something to report, and hiding it would look like the tap did nothing.

android/app/src/main/res/values/strings.xml

@@ -121,7 +121,6 @@
     <string name="update_check">Check for an update</string>
     <string name="update_install">Update</string>
     <string name="update_banner_ready">Build %1$s is downloaded and ready.</string>
-    <string name="update_banner_available">Build %1$s is available.</string>
     <string name="update_later">Later</string>
     <string name="update_failed_title">The update didn\'t install</string>
     <string name="update_permission_title">Android needs your permission</string>