fix: register src/paper/java as a Java source set on old architecture

[janicduplessis]

Summary

Fixes #754 — 5.9.0 crashes on launch on the old architecture:

java.lang.NoClassDefFoundError: Failed resolution of: Lcom/th3rdwave/safeareacontext/SafeAreaContextModule;

android/src/paper/java holds five .java files — NativeSafeAreaContextSpec.java plus the four RNCSafeArea*Manager{Delegate,Interface}.java — alongside two .kt files. The AGP 9 DSL migration in 42c50a6 (#733) registered that directory only under kotlin.directories. On some AGP/Kotlin combinations the Kotlin compiler reads those .java files for resolution but never emits class files for them, and javac never sees the directory, so they're silently dropped from the AAR. SafeAreaContextModule then can't resolve its superclass at runtime.

This adds the directory to java.directories as well, mirroring what the new-arch branch directly above already does. It's explicit either way, so it's correct regardless of which compiler picks the files up.

On reproducibility: I could not reproduce the drop with the example's toolchain (AGP 8.12.0, Kotlin 2.1.20, Gradle 9.3.1) — a clean old-arch build there produces all five classes. But it reproduces in a minimal AGP 8.12 + kotlin-android library (below), and the reporter has dexdump evidence of the classes being absent from every dex in a production AAB built with AGP 8.11.0 / RN 0.81.5 / Gradle 8.14.3. I haven't pinned down exactly which part of the toolchain flips the behavior, so I'd treat the exact trigger as unconfirmed — but the source set registration is wrong as written either way.

Worth noting android.yml does build the example with -PnewArchEnabled=false, but a missing class still compiles clean and nothing launches the app, so CI can't catch this class of regression today.

Test Plan

Minimal AGP 8.12.0 + kotlin-android library reproducing the config — a .java and a .kt in a paper-style dir, plus a Kotlin class in src/main extending the Java one:

Before (kotlin.directories only) — build succeeds, no warning:

demo/Consumer.class
demo/PaperKotlin.class      ← PaperJavaSpec.class missing

After:

demo/Consumer.class
demo/PaperKotlin.class
demo/PaperJavaSpec.class

Also verified the fix on the AGP 9.2.1 built-in-Kotlin path (where the current config instead fails loudly at compile time with Unresolved reference 'PaperJavaSpec'), and that a clean old-arch build of the example still produces a complete AAR:

$ ./gradlew :react-native-safe-area-context:assembleDebug -PnewArchEnabled=false
$ unzip -l classes.jar | grep -E "NativeSafeAreaContextSpec|Manager(Delegate|Interface)"
com/facebook/react/viewmanagers/RNCSafeAreaProviderManagerDelegate.class
com/facebook/react/viewmanagers/RNCSafeAreaProviderManagerInterface.class
com/facebook/react/viewmanagers/RNCSafeAreaViewManagerDelegate.class
com/facebook/react/viewmanagers/RNCSafeAreaViewManagerInterface.class
com/th3rdwave/safeareacontext/NativeSafeAreaContextSpec.class