Move forward installed-files.txt in the dependency graph

Bug: 5265104
- Move installed-files.txt forward to ahead of system.img in the
dependency graph, so that even if the system.img fails to build due to
too large size we can still get the size stats.
- Change the script tools/fileslist.py to sort items in decreasing size.

Change-Id: I5913bc51971e341ffbfcf3fa449c987f126e409b