Only update previous product config when changed

I'm working on removing the --ignore_dirty=$(OUT_DIR)/% argument from
Kati. If we're always writing to a file that we're also reading, then
with that flag removed, we'll always reparse all the makefiles.

So instead, use the method from the generated java file cleanup where we
read from a "previous" file, write to a "current" file, then use a
cmp || mv to overwrite the "previous" file if necessary. When the flag
is removed, this will cause Kati to need to run twice any time this
changes, but it's better than forever.

Bug: 30947985
Test: Switch between aosp_arm-eng and full-eng
Change-Id: Id2d1445809b60ce26700bb2aca765df3b5c2b360