Add features to download images over http protocol.

Add features to download images over http protocol.

- Add ResourceThreadImage::Download() to download an image using libcurl.
- Add ResoureThreadBase::RequestDownload, ResourceThreadBase::Download()
- Refactor ResoureThreadImage::Load(), Decode().
- Add one more thread object, mThreadImageRemote, to ResourceBitmapRequester to download images over http protocol in a seperate thread.
  Without this seperate thread, local image files which can be loaded very shortly may be delayed until other time-consuming remote images loading is finished.
- Employ libcurl to download files over http protocol.

Change-Id: Ie10e5b0b1fa374af74a48ad2c9f0df301d27fe5e