Fix mnn_tts build failure by copying libMNN.so to lib dir

This commit modifies `apps/Android/MnnLlmChat/build.sh` to copy the
generated `libMNN.so` to a `lib` subdirectory within the build output.
This is required because the `mnn_tts` dependency expects the library
to be located in `lib/libMNN.so`, causing build failures otherwise.

Author: google-labs-jules[bot]

apps/Android/MnnLlmChat/build.sh

@@ -24,5 +24,7 @@ cd build_64
 -DCMAKE_SHARED_LINKER_FLAGS='-Wl,-z,max-page-size=16384' \
 -DCMAKE_INSTALL_PREFIX=."
 make install
+mkdir -p lib
+cp libMNN.so lib/libMNN.so
 cd ../../../apps/Android/MnnLlmChat/
 ./gradlew assembleStandardDebug