Point the Android Java build at the installed SDK jar

ANDROID_PLATFORM means "link native code for API 28" to cargo-ndk, but the
android-build crate reads the same variable as "compile Java against
platforms/android-28/android.jar" — a directory that does not exist in the
image, because only the compile SDK is installed. Slint's Android backend
builds a Java helper through that crate, so it panicked with "No Android
platforms found" while android.jar sat in android-36.

ANDROID_JAR is checked ahead of the platform lookup and settles it: Java
compiles against the compile SDK, native code still links against MIN_API.
The two are meant to differ; only the variable name is overloaded.

Assisted-by: LLM

docker/android/Dockerfile

@@ -107,6 +107,19 @@ ENV CARGO_TARGET_AARCH64_LINUX_ANDROID_LINKER=${NDK_BIN}/aarch64-linux-android${
 ENV CARGO_NDK_PLATFORM=${MIN_API} \
     ANDROID_PLATFORM=${MIN_API}
 
+# ANDROID_PLATFORM above means "link native code for API 28" to cargo-ndk, but
+# the android-build crate reads the same variable as "compile Java against
+# platforms/android-28/android.jar" — a directory that does not exist here,
+# because only the compile SDK (ANDROID_API) is installed. Slint's Android
+# backend builds a Java helper through that crate, so it panics with
+# "No Android platforms found" while android.jar sits in android-36.
+#
+# ANDROID_JAR is checked ahead of the platform lookup and settles it: Java
+# compiles against the compile SDK, native code still links against MIN_API.
+# The two are meant to differ (see the README's compile-SDK-versus-min-API
+# note); only the variable name is overloaded.
+ENV ANDROID_JAR=${ANDROID_HOME}/platforms/android-${ANDROID_API}/android.jar
+
 # Crates with C or assembly components (ring's crypto core, and anything else
 # using the cc crate) need a compiler and archiver per target, not just a
 # linker. cargo-ndk sets the linker only, so these are set explicitly —