build: move the app to AGP 8 / Gradle 8 and AndroidX

[xroche]

The app half of the modernization. The native layer was already done; this makes Gradle and the Java side build under a current toolchain, with no runtime change at all.

AGP moves 2.3.2 to 8.6.0, Gradle 3.3 to 8.7, compileSdk 11 to 35, minSdk 9 to 24, and support-v4 to androidx.core/androidx.fragment. The dead jcenter() gives way to google()/mavenCentral() in settings.gradle. AGP 8.6 is the floor for compileSdk 35, and that pins Gradle 8.7.

targetSdk deliberately stays at master's 25, because 25 is the only value that changes nothing at runtime. I had it at 28 first, on the reasoning that anything below 29 keeps the legacy storage the File-based crawl needs. That reasoning was incomplete: 28 also crosses API 26, where a NotificationChannel becomes mandatory, and this app creates none. Every notification would have been dropped by the system with only a log warning, across four call sites including "mirror finished". Nothing user-visible, nothing a build catches. Notification channels belong with the targetSdk 35 move next phase, where an emulator can verify them.

./gradlew could not run on a clean checkout at all before this. .gitignore had /gradle, a typo for the /.gradle already on the line above, and it matched the wrapper directory, so gradle-wrapper.jar was never committed. The jar is now tracked, and the distribution is pinned by sha256.

Three things only broke once the toolchain was current. ButtonStyle set parent="@android:attr/buttonStyle", which aapt2 rejects outright because an attr is not a style, so the module cannot build under AGP 8 until it goes. aapt1 did accept it and did resolve the parent, but the only thing it inherited was the attr bag's reserved type key, which no widget ever queries, so it contributed nothing. Buttons already inherit the theme's buttonStyle as their default style, so dropping the parent leaves the appearance exactly as it has always been. OptionsActivity builds its tabs from @Fields({R.id.x}) annotations, which need R fields to be constants, hence android.nonFinalResIds=false. And unserialize(getParcelableExtra(...)) turned ambiguous, because Java 8+ can infer T as File & Parcelable and match the unserialize(File) overload too.

One deviation from the plan worth checking in review: it said to rename the FileProvider FILE_PROVIDER_PATHS meta-data to an androidx key, but androidx.core.content.FileProvider still reads android.support.FILE_PROVIDER_PATHS. I checked that against the compiled 1.13.1 artifact. Renaming it would have broken sharing at runtime with nothing in the build to catch it, so the key is unchanged.

./gradlew clean assembleDebug assembleRelease lint is green against a local SDK 35 / NDK r26d install, and both APKs carry arm64-v8a and x86_64 only, targetSdkVersion 25, versionCode 63. AGP 8's packaging flip is fine here: the .so entries are stored uncompressed with extractNativeLibs=false, which needs API 23+ and minSdk is 24, and they come out 16 KB page aligned. Depending on modern androidx.core also pulls profileinstaller in, so the merged manifest gains androidx.startup.InitializationProvider and a ProfileInstallReceiver gated behind the DUMP permission; worth an ack rather than a surprise.

Lint is non-fatal (abortOnError false) and reports 13 pre-existing errors, mostly the OptionsActivity$*Tab manifest entries that are registered as activities but extend Tab; that wants its own PR. Nothing was installed on a device: there is no KVM access on this machine, so the emulator smoke job still waits for Phase 4.