[ALOY-108] tss file expr() syntax needs to be handled recursively

Problem

expr() syntax in tss files currently only works in a flat style definition with no nested objects. For example, the following style definition will parse the width correctly, but will not properly parse the fontSize:

".container": {
    "width": expr(Ti.Platform.displayCaps.platformWidth/2),
    "font": {
        "fontSize": expr(require('alloy/measurement').dpToPx(18))
    }
}

Proposed Solution

Processing of style attributes needs to be performed recursively. Right now, if an object type is encountered, it is simply returned as-is with no expr() evaluations. We need to pass the object back into the style parsing function to further process it to allow expr() syntax to work.