[PromptText] Styling Spark DropDownList prompt text isnt consistent w/ ComboBox, TextArea, and/or TextInput

Found in 4.5.0.18027

Steps to reproduce:
1. Compile and run attached MXML file.
2. Set your eyes on the prompt text in the s|DropDownList control

Actual Results:
"MY EYES!!"

Expected Results:
Styling the s|DropDownList prompt text should be consistent with the styling of s|ComboBox, s|TextArea, and s|TextInput.
Although this could introduce some slight backwards compat issues since s|DropDownList has always supported prompt text.

Workaround (if any):
n/a