The bestmaxFrameSizeForScreenSize() function is full of magic numbers.
(There originally, in OOo times, was a comment in there that said
"fill in holy default values brought to us by product management"
which says it all, really.) Those numbers have little or no relevance
on iOS. Instead, make this function return the largest square that
will fit on the display (square so that it will fit in either
orientation, in case the device is rotated while a tunnelled dialog is
displayed).

And as a consequence the defaut font size on iOS can be bumped up a
bit, to 10. Now the problematic dialogs look even better.

Note that this is a cherry-pick from our vendor branch (cp-6.0). It
hasn't actually been tested as the iOS app doesn't work at the moment
if built from the master branches of online and core.

Change-Id: I043d8f9947520adb04bd952ee49f9c7844a1fa8c