Gradient for VectorDrawable's fill and stroke

Add ComplexColor interface for both GradientColor and ColorStateList.
Set up constant state, factory, theme attrs for GradientColor, while
refactoring the ColorStateList's similar code. (Functionality in CSL should
be the same).

Support themeing in both the root and item level in GradientColor.
For example, both startColor in <gradient> tag or color in <item> tag can
have theme color.
Add tests for both simple and complex cases with themeing etc.

Hook up the native VectorDrawable implementation using 2 extra JNI calls for
simplicity. Such calls only happen at inflate and applyTheme call.

b/22564318

Change-Id: Ibdc564ddb4a7ee0133c6141c4784782f0c93ce0e