android: Exclude all data from backup

android/app/src/main/AndroidManifest.xml

@@ -7,7 +7,11 @@
    <application
         android:label="Zulip"
         android:name="${applicationName}"
-        android:icon="@mipmap/ic_launcher">
+        android:icon="@mipmap/ic_launcher"
+        android:allowBackup="false"
+        android:fullBackupContent="@xml/full_backup_content"
+        android:dataExtractionRules="@xml/data_extraction_rules"
+        >
         <activity
             android:name=".MainActivity"
             android:exported="true"

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

@@ -0,0 +1,47 @@
+<?xml version="1.0" encoding="utf-8"?>
+<!-- Exclude all data from device backup.  For motivation, see:
+       https://github.com/zulip/zulip-flutter/issues/2158
+       https://github.com/zulip/zulip-flutter/issues/2159
+     The data that's essential to exclude is in our database,
+     at `domain="file" path="zulip.db"`.  But once that's excluded,
+     including any stray other data would at best cause a confusing
+     mixed state, if it has any effect at all.  So exclude it all.
+
+     Docs for this file:
+       https://developer.android.com/identity/data/autobackup#include-exclude-android-12
+  -->
+<data-extraction-rules>
+    <cloud-backup>
+        <exclude domain="file" />
+        <exclude domain="database" />
+        <exclude domain="sharedpref" />
+        <exclude domain="external" />
+        <exclude domain="root" />
+        <exclude domain="device_file" />
+        <exclude domain="device_database" />
+        <exclude domain="device_sharedpref" />
+        <exclude domain="device_root" />
+    </cloud-backup>
+    <device-transfer>
+        <exclude domain="file" />
+        <exclude domain="database" />
+        <exclude domain="sharedpref" />
+        <exclude domain="external" />
+        <exclude domain="root" />
+        <exclude domain="device_file" />
+        <exclude domain="device_database" />
+        <exclude domain="device_sharedpref" />
+        <exclude domain="device_root" />
+    </device-transfer>
+    <cross-platform-transfer platform="ios">
+        <exclude domain="file" />
+        <exclude domain="database" />
+        <exclude domain="sharedpref" />
+        <exclude domain="external" />
+        <exclude domain="root" />
+        <exclude domain="device_file" />
+        <exclude domain="device_database" />
+        <exclude domain="device_sharedpref" />
+        <exclude domain="device_root" />
+    </cross-platform-transfer>
+</data-extraction-rules>

android/app/src/main/res/xml/full_backup_content.xml

@@ -0,0 +1,29 @@
+<?xml version="1.0" encoding="utf-8"?>
+<!-- Exclude all data from device backup.
+     For background, see `data_extraction_rules.xml`.
+
+     It's not clear this file has any effect: for Android 12 and up,
+     the `data_extraction_rules.xml` file
+     (`android:dataExtractionRules` in the manifest)
+     is supposed to take precedence.
+     And for Android 11 and older, the `android:allowBackup="false"`
+     which we also set is supposed to prevent backups completely.
+
+     But including this extra file is cheap.  And from experience,
+     we don't trust Android's documentation to be 100% accurate;
+     maybe there's some situation where it helps after all.
+
+     Docs for this file:
+       https://developer.android.com/identity/data/autobackup#include-exclude-android-11
+  -->
+<full-backup-content>
+    <exclude domain="file" />
+    <exclude domain="database" />
+    <exclude domain="sharedpref" />
+    <exclude domain="external" />
+    <exclude domain="root" />
+    <exclude domain="device_file" />
+    <exclude domain="device_database" />
+    <exclude domain="device_sharedpref" />
+    <exclude domain="device_root" />
+</full-backup-content>