Fix drawable CTS breakages

1. ClipDrawable.getOpacity() now correctly respects the current level
2. DrawableWrapper checks the contained drawable's changing config even
   if it doesn't have a constant state
3. DrawableWrapper gives priority to the last valid child drawable
   rather than the drawable attribute

Bug: 21406104
Change-Id: I442fe90d0a3865bfdc6b2d14a7358178310dde02