Fix build script

Author: topjohnwu

app/.gitignore

@@ -3,7 +3,6 @@
 /local.properties
 .idea/
 /build
-app/release
 *.hprof
 .externalNativeBuild/
 *.apk

build.py

@@ -429,7 +429,7 @@ def build_app(args):
     # build process. Copy the stub APK into output directory.
     build_type = 'release' if args.release else 'debug'
     apk = f'stub-{build_type}.apk'
-    source = op.join('app', 'src', 'main', 'assets', 'stub.apk')
+    source = op.join('app', 'src', build_type, 'assets', 'stub.apk')
     target = op.join(config['outdir'], apk)
     cp(source, target)
 
@@ -457,7 +457,8 @@ def cleanup(args):
     if 'java' in args.target:
         header('* Cleaning java')
         execv([gradlew, 'app:clean', 'app:shared:clean', 'stub:clean'])
-        rm_rf(op.join('app', 'src', 'main', 'assets'))
+        rm_rf(op.join('app', 'src', 'debug'))
+        rm_rf(op.join('app', 'src', 'release'))
 
 
 def setup_ndk(args):