update Text gradient colors to be mirrored on ios

[timothy-huynh]

Summary:

When we added gradient colors to Text in #54, we forgot to duplicate the first color to create a mirrored pattern. Without the duplicated first color, the color transitions abruptly from the last color back to the first color.

Changelog:

• duplicate first color when we create gradient color for Text

Test Plan:

• Tested the chat input context bar which is the only component actively using this gradient

before

after