ThemeChooser : Scale iconSize in previews

- Use FIT_CENTER to ensure the icon is scaled within bounds
- Set weight property to ensure even space distribution
- Set default icon size on the resulting icon drawable

Change-Id: I83ff2bac5bf0aad739f39e0c0b4aff20c231c5d4
Merge "ThemeChooser : Scale iconSize in previews" into cm-11.0