Fix handling of non-integer ANDROID_PLATFORM

ANDROID_PLATFORM is not always an integer; it can also be a value like
"36.1". Handle such cases properly.

This fixes the following error:

    server/build_without_gradle.sh: line 89:
    [[: 36.1: syntax error: invalid arithmetic operator (error token is ".1")

PR #6408 <https://github.com/Genymobile/scrcpy/pull/6408>

Signed-off-by: Romain Vimont <rom@rom1v.com>

server/build_without_gradle.sh

@@ -86,7 +86,7 @@ javac -encoding UTF-8 -bootclasspath "$ANDROID_JAR" \
 echo "Dexing..."
 cd "$CLASSES_DIR"
 
-if [[ $PLATFORM -lt 31 ]]
+if [[ "${PLATFORM%%.*}" -lt 31 ]]
 then
     # use dx
     "$BUILD_TOOLS_DIR/dx" --dex --output "$BUILD_DIR/classes.dex" \