fix: keep pending crash reports out of Auto Backup

The backup rules were empty, despite their comments, so a pending crash
report followed a restore to a new phone and prompted there.

app/src/main/res/xml/backup_rules.xml

@@ -1,4 +1,8 @@
 <?xml version="1.0" encoding="utf-8"?>
 <full-backup-content>
-    <!-- No file-based backups; settings live in DataStore which is backed up by default. -->
+    <!-- Everything is backed up (the database and DataStore) except a pending
+         crash report, which belongs to the device it happened on and would
+         otherwise prompt again after a restore. Keep in step with
+         data_extraction_rules.xml. -->
+    <exclude domain="file" path="crash/" />
 </full-backup-content>

app/src/main/res/xml/data_extraction_rules.xml

@@ -1,8 +1,10 @@
 <?xml version="1.0" encoding="utf-8"?>
 <data-extraction-rules>
+    <!-- As backup_rules.xml: everything but a pending crash report. -->
     <cloud-backup>
-        <!-- Allow DataStore backup, exclude nothing extra. -->
+        <exclude domain="file" path="crash/" />
     </cloud-backup>
     <device-transfer>
+        <exclude domain="file" path="crash/" />
     </device-transfer>
 </data-extraction-rules>