sdcardfs: Truncate packages_gid.list on overflow

packages_gid.list was improperly returning the wrong
count. Use scnprintf instead, and inform the user that
the list was truncated if it is.

Bug: 30013843
Change-Id: Ida2b2ef7cd86dd87300bfb4c2cdb6bfe2ee1650d
Signed-off-by: Daniel Rosenberg <drosen@google.com>