Correctly propagate permissions when uninstalling updates.

In ag/733689, which was intended to fix this bug, the following lines
were removed:

// Propagate the permissions state as we do want to drop on the floor
// runtime permissions. The update permissions method below will take
// care of removing obsolete permissions and grant install permissions.
ps.getPermissionsState().copyFrom(disabledPs.getPermissionsState());

The intent with these lines seemed to be that we needed to copy
permissions from the application on /data, which is being uninstalled,
over to the copy on /system, which was disabled but is being
reenabled. However, it wasn't functional, because it incorrectly
copied from the copy on /system, not the copy on /data.

Restore this code, copying from newPs (the copy on /data) rather than
disbledPs (the copy on /system), and clarify the comment because we do
*not* want to drop runtime permissions on the floor.

Bug: 22665508
Change-Id: I6bae37e70b6df1043c9a2b49255b985707ba151a