Fixing download_dependencies.sh bugs for generating TFLite iOS exmaples (#14734)

* Fix: Can't build TFLite after running download_dependencies.sh.

Root cause: The script downloads files for building TFLite for iOS
example. It writes to `downloads/` directory and conflicts with the
visibility rule "**/*" in BUILD

* Retain lite/examples/ios/camera/data directory in git.

* Fix some bugs in download_dependencies.sh

* Handle both the cases that the zip file has nested directories
  or not.
* Always use `curl` since `wget` sometimes has certificate problem
  in some Mac machines.