[TIMOB-17726] iOS: Support Ti.Platform.displayCaps.logicalDensityFactor

JIRA Ticket: https://jira.appcelerator.org/browse/TIMOB-17726
On iOS devices, this property returns 1, 2 and 3 for @1x, @2x and @3x respectively. 
Note for iPhone 6+, this value is 3.