Make the Android app build, and fix what compiling it exposed

Why: the module was written but never compiled, so nothing about it was
proved. Building it found a name clash and a theme that would have crashed
every device below Android 10.

What:
- KeepOrgProvider.requireContext -> ctx(): ContentProvider gained
  requireContext() in API 30, and Kotlin refused the silent hiding.
- The application theme was @android:style/Theme.DeviceDefault.DayNight,
  which is API 29 while minSdk is 26. Replaced with @style/AppTheme, plain
  DeviceDefault in values/ and DayNight in values-v29/.
- Backup exclusions: data_extraction_rules.xml and backup_rules.xml exclude
  every domain for both cloud backup and device transfer. The session token
  authenticates as the user and the mirrored files are the organization's
  password databases; neither belongs in a backup.
- User-facing strings moved from code into strings.xml, so the app is
  translatable. Layouts stay in code, as before.
- mipmap-anydpi-v26 -> mipmap-anydpi (the qualifier is redundant at minSdk
  26), and the redundant activity label is gone.
- Lint is configured to write a text report, and `make android-lint` runs it;
  `make android` now fails with a clear message when ANDROID_HOME has no SDK.

Proved: the unsigned release APK builds (656 KB), lint reports "No issues
found", and the packaged DEX contains all 16 classes including every manifest
component. Toolchain used: JDK 21, Gradle 8.10.2, SDK platform 35 and
build-tools 35.0.0; on this arm64 host Google's x86-64 aapt2 runs under
qemu-user-static with amd64 multiarch libraries, which android/README.md now
documents. The app has still not run on a device: there is none here.

android/app/src/main/AndroidManifest.xml

@@ -8,14 +8,15 @@
 
     <application
         android:allowBackup="false"
+        android:dataExtractionRules="@xml/data_extraction_rules"
+        android:fullBackupContent="@xml/backup_rules"
         android:icon="@mipmap/ic_launcher"
         android:label="@string/app_name"
-        android:theme="@android:style/Theme.DeviceDefault.DayNight">
+        android:theme="@style/AppTheme">
 
         <activity
             android:name=".MainActivity"
-            android:exported="true"
-            android:label="@string/app_name">
+            android:exported="true">
             <intent-filter>
                 <action android:name="android.intent.action.MAIN" />
                 <category android:name="android.intent.category.LAUNCHER" />