Add Hilt testing manifest activity artifact

[loganrosen]

Fixes #3394.

Summary

• Add com.google.dagger:hilt-android-testing-manifest, a small published AAR with dagger.hilt.android.testing.HiltComponentActivity and a manifest activity entry for Hilt Android tests.
• Wire Gradle/Bazel publishing, artifact validation, release deploy, API tracking, and coalesced Javadocs.
• Add a consumer-shaped Hilt fixture instrumentation test that launches the activity from the manifest artifact and verifies ActivityComponent access.

Validation

• :hilt-android-testing-manifest:assembleDebug
• :hilt-android-testing-manifest:apiCheck
• :hilt-android-testing-manifest:publishToMavenLocal
• bazelisk build --noshow_progress //hilt-android-testing-manifest:artifact
• bazelisk build --noshow_progress //:user-docs
• Hilt simple fixture :app:assembleFreeDebug, :app:assembleFreeDebugAndroidTest, :app:assembleProDebug, :app:assembleProDebugAndroidTest
• :app:connectedFreeDebugAndroidTest filtered to dagger.hilt.android.simple.HiltComponentActivityTest
• Scratch Now in Android replacement trial with focused Robolectric tests against the new artifact

[loganrosen]

@Chang-Eric would you be the right person to review this, or could you redirect me to the current Hilt Android reviewer? This implements the convenience Hilt test activity discussed in #3394.

[bcorso]

@loganrosen I can review this.

One high-level question here -- is it required/preferred for this to be a separate artifact rather than putting it in the existing hilt-android-testing artifact?

[loganrosen]

@bcorso thanks, I appreciate it.

I don't think a separate artifact is strictly required, but I chose it to keep the dependency shape narrow. The activity's manifest entry has to be merged into the target app/debug APK, so consumers need this on debugImplementation or equivalent. Their normal androidTestImplementation("hilt-android-testing") dependency would not contribute the activity declaration to the app manifest.

If we put this in hilt-android-testing, consumers could add that artifact to debugImplementation, but that would also pull the full Hilt testing API/dependency surface into the app variant just to get one manifest activity. For example, hilt-android-testing currently exposes test-facing deps like JUnit and AndroidX Test Core as API deps.

The separate artifact is meant to mirror the AndroidX ui-test-manifest pattern: app/debug gets only the reusable activity + manifest entry, while tests keep using hilt-android-testing for HiltAndroidRule, @HiltAndroidTest, etc. That said, I'm happy to fold it into hilt-android-testing if you prefer that API shape.