Restructure notification channels by intent and importance

Notifications were too intrusive: install-success and export sat on
IMPORTANT_HIGH channels and produced heads-up alerts for every app,
while genuine download failures were buried in the IMPORTANCE_MIN
downloads channel where they were easily missed.

Restructure the channels around user intent:

- Download progress -> MIN, silent, no badge
- Installed apps    -> LOW, silent (was HIGH)
- Updates available -> DEFAULT, silent
- App export        -> LOW, silent (was HIGH)
- Errors & alerts   -> HIGH (new, the only heads-up channel)

Failed downloads, installer-status errors and the unarchive auth prompt
now route to the high-importance alerts channel so they aren't lost,
while successful installs/exports stay quiet.

Android ignores importance edits on an already-created channel, so the
install/export channels get new IDs and the retired IDs (including the
folded-in account channel) are deleted on next launch.

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

@@ -211,7 +211,7 @@
     <string name="device_miui_extra">"Optionally you can choose Native installer, but then you can not install bundled (split) APKs, so choice is yours."</string>
     <string name="dialog_title_self_update">"New update available"</string>
     <string name="dialog_desc_native_split">"You can not install bundled (split) apps via Native Installer. Change your installer to Session, Services or Root."</string>
-    <string name="notification_channel_account">Account-related notification</string>
+    <string name="notification_channel_alerts">Errors &amp; alerts</string>
     <string name="notification_channel_export">App export notification</string>
     <string name="notification_channel_install">Install notification</string>
     <string name="notification_channel_downloads">Downloads notification</string>