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/java/com/beemdevelopment/aegis/ui/EditEntryActivity.java

@@ -467,7 +467,7 @@ public class EditEntryActivity extends AegisActivity {
         intent.putExtra("entryUUID", entry.getUUID());
         intent.putExtra("delete", delete);
 
-        if (saveVault()) {
+        if (saveVault(true)) {
             setResult(RESULT_OK, intent);
             finish();
         }