Theme changes for flyout

Flyout radius / background wasn't being updated on theme change. This
CL fixes that.

Test: manual - 1) post a bubble, note the flyout, toggle dark mode, post a
                  bubble, note the flyout background & text colour is
                  accurate
               2) post a bubble, note the flyout, change theme to something
                  with different radius (e.g. circle -> square icons),
                  post a bubble, note the flyout radius matches the theme
Bug: 138749133
Change-Id: I74d9dedc5099f6b24eaf9a90ff4fb1a22db9d7d8