Layout may be called with invalid print attributes.

If a print service reports a printer with no capabilities and then
removes and adds the same printer again with no capabilities we can
call layout with invalid attributes. While the print service should
not do that we must not crash. The reason is that the print dialog
code was lacking a check whether something changed before handling
the change of the current printer. This resulted in calling layout
even though we still do not have the printer capabilities resulting
in passing bogus print attributes to the app.

bug:11216402

Change-Id: Icb9161930a70a7f9ead589a0ab0b4eb35091666f