Extract ANDROID_JAR path in build script

This will allow to reuse it.

server/build_without_gradle.sh

@@ -21,6 +21,7 @@ BUILD_DIR="$(realpath ${BUILD_DIR:-build_manual})"
 CLASSES_DIR="$BUILD_DIR/classes"
 SERVER_DIR=$(dirname "$0")
 SERVER_BINARY=scrcpy-server
+ANDROID_JAR="$ANDROID_HOME/platforms/android-$PLATFORM/android.jar"
 
 echo "Platform: android-$PLATFORM"
 echo "Build-tools: $BUILD_TOOLS"
@@ -47,8 +48,8 @@ cd "$SERVER_DIR/src/main/aidl"
 
 echo "Compiling java sources..."
 cd ../java
-javac -bootclasspath "$ANDROID_HOME/platforms/android-$PLATFORM/android.jar" \
-    -cp "$CLASSES_DIR" -d "$CLASSES_DIR" -source 1.8 -target 1.8 \
+javac -bootclasspath "$ANDROID_JAR" -cp "$CLASSES_DIR" -d "$CLASSES_DIR" \
+    -source 1.8 -target 1.8 \
     com/genymobile/scrcpy/*.java \
     com/genymobile/scrcpy/wrappers/*.java