android: find updates without being asked, fetch them, then nag

`check()` had exactly one caller: a button on the sync screen. So a new build was
found only by someone who went looking for one — and having to remember to go
looking is the same as not being told. The operator has been doing that by hand
every time.

Three parts.

FIND. The app checks when it comes forward, which is the moment the person is
present. Rate-limited to six hours in the view model, so flicking between two apps
is not a re-check, and skipped entirely on an unlinked device — updates come from a
linked server and there is nothing to ask. Same ForegroundTransitions shape as
AutomaticSync, for the same reason.

FETCH. Finding one downloads it, so the nag is a one-tap install rather than the
start of a wait. NOT over mobile data: fifty-odd megabytes is a bill nobody agreed
to, so this is gated on an unmetered connection (new ACCESS_NETWORK_STATE
permission — normal, no prompt). On a metered link the update is still found and
still nags; Install downloads it then, which is a choice rather than a surprise.

NAG. A banner on the board, under the error banners — an update is worth saying and
never worth saying before a note failed to save. "Later" clears it for this sitting
only: the next time the app comes forward the check finds the same build and says so
again. That is the difference between a reminder and a notice you can lose.

downloadAndInstall now skips the download when the background fetch already did it,
so the sync screen's button and the banner's are the same action with the same
name — whether the bytes are already there is this class's problem, not the
person's.

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

@@ -120,6 +120,9 @@
     <string name="update_current">You\'re on the newest build this server has.</string>
     <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>
     <string name="update_permission_body">ThoughtSync has to be allowed to install apps before it can update itself. This is a one-time setting.</string>