Platform logo activity font size adjustment

Font size of platform logo's version letter (eg., 'K' for KitKat)
must be set according to the Display density / resolution.

Until now the size is hardcoded to '300sp' in code. Now setting the
value through 'dimen' resource, whose value is overridden through
the 'overlay' approach, based on device

Change-Id: I5ad1c31b46507ce6c2977c4448513fb20b7c046e