Remove @hide from android.graphics.Color

Bug: 156505010
Test: HtmlTest
Test: I060478a4e46f4ed153007d42cb5b7c5eae04de27

Remove Color#getHtmlColor, which was marked as @hide. Its only caller
was outside the in-progress UI rendering module, so it would no longer
be able to access it after the move.

Make the calling code call either the already-public Color#parseColor or
the (@hide, which is okay from android.text) XmlUtils.convertValueToInt
directly to achieve the same effect as before.

Change-Id: I8fde97bb866689849ba49932c6d7ca10bbd618c5