Improve fix for bug #8159072 Spinner widget should be RTL'ized

- populate the preloaded drawable cache with only LTR drawables
when the layout direction during preloading is LTR. Populate
the cache with only RTL drawables when the layout direction during
preloading is RTL
- only preload drawables that dont have a dual LTR/RTL version

Change-Id: I7807bdc031b99102609efda75042a9500d96065c