[lib] Introduce SelectedThreadColors type

Summary: Not sure I'm doing this right... but wanted to create a Flow type that was basically a union of the strings in selectedThreadColors without writing everything out twice.

Test Plan:
Seems to work as expected: https://flow.org/try/#0PQKgBAAgZgNg9gdzCYAoVBjOA7AzgFzFwFMZiN9iATAFQAsAnYgQyoGE54HcwBeMAN6owYAOQAhABwB2AKwBmACKiAXGACMAGmFi2k2QAYjqsACZtI0QEErAFnH2T8i2PsybJ2y9EA2W5LYPNVlvWWkfAE5pADETH28fRVsIq3ETaW8jK1kI9RNJTINJaMl5EwjQuSqTdQNtAF8AbnRiAA8ABzgGQnwAT3biMABlUnJKWkYWdk4unn4AEgBpYl7cAB4+gbgoIlGKanomVg4uXAA+ZswcAjBmNRGyfYmj6dO+MSNi0tFmrDxCABG9z240OUxOs3eogAxJ8SmVGkA

Also introduced type to ColorSelector and it worked as expected.

Reviewers: palys-swm, def-au1t, ashoat

Reviewed By: ashoat

Subscribers: Adrian, karol-bisztyga, benschac, yayabosh

Differential Revision: https://phabricator.ashoat.com/D3679