Android: Scan on background thread and show progress

android/app/src/cpp/CMakeLists.txt

@@ -1,6 +1,8 @@
 set(SRCS
   android_host_interface.cpp
   android_host_interface.h
+  android_progress_callback.cpp
+  android_progress_callback.h
   android_settings_interface.cpp
   android_settings_interface.h
 )