Resurrect HelloAndroid and HelloAndroidCamera2

[alexreinking]

apps/HelloAndroid had no working build: build.gradle pinned Gradle 2.2 / AGP 1.2.2 and resolved from jcenter(), which shut down in 2021; jni/Application.mk's APP_STL := gnustl_static is a hard error on current NDKs; and the src/res/AndroidManifest.xml-at-the-root layout predates Android Studio.

Replace it with the standard app/src/main/{java,res,cpp} module layout, current Gradle/AGP, and a single app/src/main/cpp/CMakeLists.txt that Gradle's externalNativeBuild drives directly. The CMakeLists.txt configures itself twice: once as a host build (to compile the Halide generator, via execute_process() before find_package() needs it) and once as this Android target build. This is admittedly a hack to work around Gradle's lack of a host-target externalNativeBuild variant. Targets arm64-v8a and x86_64, dropping 32-bit Android targets that are obsolete.

Also fixes two runtime bugs: CameraActivity never requested the CAMERA permission at runtime (added the standard ContextCompat/ActivityCompat check-then-request flow), and hello_wrapper.cpp built its halide_buffer_t structs without setting .dimensions, so every pipeline call failed a dimension-count check and the app eventually crashed.

HelloAndroidCamera2 got the same modernization and also fixed three runtime issues: CameraActivity never requested the CAMERA permission at runtime (added the standard ContextCompat/ActivityCompat check-then-request flow); Camera2BasicFragment.java's switch (view.getId()) { case R.id.toggle: } doesn't compile under AGP's non-final resource IDs default, rewritten as an if; and YuvBufferT's "packed planar" chroma case built a Buffer for just the U (or V) plane and then called crop(1, 0, height * 2) to try to extend it to cover both planes -- Buffer::crop() only ever narrows within a buffer's existing declared bounds, so this crashed (SIGABRT) the moment the camera produced a frame in this layout. Fixed by constructing packedPlanarChromaView_ directly from the pointer with the full doubled-height shape, the same way the sibling "interleaved" branch already does.

Testing

Add an android-apps job (testing-linux.yml) that builds both apps via ./gradlew assembleDebug and then smoke-tests them (apps/support/android_smoke_test.sh) against a real booted emulator, rather than only checking that they build: (1) fresh install, launch, confirm the OS shows a runtime permission prompt -- catches an app relying on its manifest declaration alone; (2) grant the permission, relaunch, and confirm the process survives with none of a set of known-bad logcat signatures (fatal crash signals, plus each app's own previously-fixed bug's exact error string). Deliberately does not require seeing a "pipeline completed" log line: whether the camera preview surface negotiates a size the pipeline actually runs on is sensitive to the emulator's screen/camera configuration, which varies by
environment.

Uses reactivecircus/android-emulator-runner with an x86_64 (not arm64-v8a) system image so the emulator gets KVM hardware acceleration on these x86_64 GitHub-hosted runners, and an emulated back/front camera so no physical camera is needed.

Fixes #8727
Fixes #8728

Breaking changes

None.

Checklist

• Tests added or updated (not required for docs, CI config, or typo fixes)
• Documentation updated (if public API changed)
• Commits include AI attribution where applicable (see Code of Conduct)

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
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Additional details and impacted files

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[abadams]

I recommend just deleting HelloAndroid and renaming HelloAndroidCamera2 to HelloAndroid