Follow up on issue #8159072: Spinner widget should be RTL'ized

Only allow through changing direction configs for drawables.

Explicitly map layout direction values to an index in the
preload arrays.

Drawables that don't vary by configuration should go in to both
the rtl and ltr preloads.

Change-Id: Ib92dd11738082a795e02d1d4191adb54702d651c