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/build.gradle.kts

@@ -34,6 +34,11 @@ android {
     kotlinOptions {
         jvmTarget = "17"
     }
+    lint {
+        // A lint finding is a defect until it is fixed or recorded here.
+        textReport = true
+        textOutput = file("build/reports/lint-results.txt")
+    }
     dependenciesInfo {
         // Keep the APK free of the Google dependency metadata blob so the
         // build is reproducible for F-Droid.