Updated GradientDrawable to expose public APIs to

reduce reflection usages

--Marked GradientState parameter as hidden as each field on it is
accessible through GradientDrawable's public API
--Updated setColors API method to enable setting color stop positions
array
--Fixed issue where GradientDrawable with start/center/end colors
configured via xml would crash at runtime if the colors were modified
with a different number of colors. Resolved by providing optional
float array parameter that defaults to null if unset.
--Added public setters to GradientState parameters that were not
previously exposed

Bug: 124051827
Bug: 124051738
Bug: 124050218
Bug: 124050917
Test: Added CTS tests to GradientDrawableTest for new API coverage
Change-Id: I175998c9920d0ecca021fc3cac7d3a3f9e4b61cf