Group install and failure notifications under summaries

A bulk update produced one standalone notification per app for both
successes and failures, flooding the shade. Bundle these terminal
notifications into two groups, each with a collapsible summary that
lists the apps via InboxStyle:

- "N apps installed" on the (LOW, silent) installed channel, expiring
  on its own since it's informational.
- "N apps failed" on the (HIGH) alerts channel, persisting until
  handled and carrying a Retry action per app.

Summaries are rebuilt from the live posted notifications rather than
tracked state, so they stay correct across retries, dismissals and
process restarts, and are only posted on Android N+ where the system
actually renders groups.

Retry re-queues the download via a new DownloadRetryReceiver; the
worker resumes from verified files on disk, so retrying an install
failure re-installs without re-downloading.

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

@@ -576,6 +576,14 @@
         <item quantity="one">Permission required</item>
         <item quantity="other">Permissions required</item>
     </plurals>
+    <plurals name="notification_installed_summary">
+        <item quantity="one">%d app installed</item>
+        <item quantity="other">%d apps installed</item>
+    </plurals>
+    <plurals name="notification_failed_summary">
+        <item quantity="one">%d app failed</item>
+        <item quantity="other">%d apps failed</item>
+    </plurals>
 
     <!-- AppsGamesFragment -->
     <string name="installed_apps_size"><xliff:g id="installed_apps_size">%1$d</xliff:g> apps installed</string>