android.sh: fix flavor default ([[ $flavor ]], not literal 'flavor')

The empty-flavor test compared the literal string 'flavor', so a no-flavor
invocation left the APK output path as apk//debug/app--debug.apk and the
rename failed. Default to 'local' correctly.

scripts/android.sh

@@ -61,7 +61,7 @@ function build_lib() {
 ### Build application
 function build_apk() {
   flavor=$3
-  [[ flavor == "" ]] && flavor="local"
+  [[ -z "$flavor" ]] && flavor="local"
   cd android || exit 1
   ./gradlew clean
   # Build signed apk if keystore exists