Update thumbnail when file is updated

When a file is updated, even if it's different content but the same
file name, the thumbnail file is not updated. We need to renew the
thumbnail for another content that uses an existing file name.
"Media.MINI_THUMB_MAGIC" should be set to 0 to trigger creation/re-creation
of thumbnail in MediaProvider.

Change-Id: Iaf5a0ea0c1069a0d7b3b47ee89c2357793278c17