====== Android APK backup ======
===== Locate the package name =====

  adb shell pm list packages
===== Locate the apk path =====

  adb shell pm path com.google.android.youtube
  
===== adb pull out the APK file =====
  adb  pull  /data/app/com.google.android.youtube-1/base.apk .
If it says "xxx does not exist", then you can try copy the file to sdcard
  adb shell
  HWEVA:/ $ cp /data/app/com.google.android.youtube-1/base.apk /sdcard/
Then pull out the APK file
   adb pull /sdcard/base.apk .