Persist most recent tinker color on all edit views

This issue has been migrated from mozilla/webmaker-android#2204.

It was originally written by xmatthewx on Fri Jun 12 2015 and had the following description:

Scenario: user adds a custom color for a header on a page. They want to use this color on other pages as well. They need to recreate the color using tinker mode. (I guess it gives them a reason to memorize hex or rgb values). Ideally, most recent tinker color persists in all edit views.

On Sat Jun 13 2015, LauraReynal commented:

I would tend to see if they memorize the numbers.

I feel the same thing when I play with tinker mode, but I like the challenge, as a user.