Speed up Partition::Update_Size() for devices with /data/media

* Skip /data/media when walking through the tree instead of getting
  size of whole /data, then getting size of /data/media (hence
  scanning /data/media twice) and substracting those values.
* /data/media can often be the biggest and deepest folder, so this
  can improve TWRP's initialization speed significantly.

Signed-off-by: Vojtech Bocek <vbocek@gmail.com>

Change-Id: I9bd54b25ee09c6a5ed32f521e06dff9f5d6a81d9