Clean up LayerDrawable, refactoring only

Uses consistent naming and ordering for padding and inset calculations.
Updates documentation to include UNDEFINED_INSET as public API, since we
return the value and will accept the value as a valid param. Flattens
out RTL logic for readability.

Change-Id: Ie704e2e5fc7f5763500ebe1217ddbbf4d0c5ef86