applypatch: Remove the 'st' field from FileContents.

It used to keep track of the stat(2) info (e.g. st_mode/st_gid/st_uid)
while patching a file in file-based OTA.

Test: Build and use the new updater to apply an update on bullhead.
Change-Id: Ibf8f0f4b14298a9489bf24a2678bb279c5d9c8f3