Improve backup error handling and frequency

This patch improves our backup functionality in a number of ways:
- Only backup the vault when important changes are made, not when the order of
  entries is changed, for instance.
- Don't bubble up backup errors when saving the vault.
- Instead, show an error bar in the main view if the most recent backup attempt
  failed.

<img src="https://alexbakker.me/u/kbhhj2hcgx.png" width="300" />

Clicking on the error bar will take the user to the backup settings.

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

@@ -248,6 +248,7 @@
     <string name="google_qr_export_unrelated">Unrelated QR code found. Try restarting the scanner.</string>
     <string name="google_qr_export_scanned">Scanned %d/%d QR codes</string>
     <string name="google_qr_export_unexpected">Expected QR code #%d, but scanned #%d instead</string>
+    <string name="backup_error_bar_message"><b>Vault backup failed recently</b></string>
 
     <string name="custom_notices_format_style" translatable="false" >
         body {