fix: use Adoptium APT repo for JDK 21 in Android Dockerfile

Bookworm only ships JDK 17. Add Adoptium's official APT repository
to install Temurin 21 via package manager.

Dockerfile.android

@@ -1,10 +1,13 @@
 FROM rust:1.87-bookworm
 
-# Java 21
+# Java 21 via Adoptium APT repo (Bookworm only has JDK 17)
 RUN apt-get update && apt-get install -y --no-install-recommends \
-    openjdk-21-jdk-headless unzip wget curl && \
+    unzip wget curl ca-certificates gnupg && \
+    wget -qO- https://packages.adoptium.net/artifactory/api/gpg/key/public | gpg --dearmor -o /usr/share/keyrings/adoptium.gpg && \
+    echo "deb [signed-by=/usr/share/keyrings/adoptium.gpg] https://packages.adoptium.net/artifactory/deb bookworm main" > /etc/apt/sources.list.d/adoptium.list && \
+    apt-get update && apt-get install -y --no-install-recommends temurin-21-jdk && \
     rm -rf /var/lib/apt/lists/*
-ENV JAVA_HOME=/usr/lib/jvm/java-21-openjdk-amd64
+ENV JAVA_HOME=/usr/lib/jvm/temurin-21-jdk-amd64
 
 # Android SDK
 ENV ANDROID_HOME=/opt/android-sdk