Only log theme change events if the theme was actually changed.

Right now the MobileWikiAppAppearanceSettings schema records each tap on the
dialogue for changing themes. This patch changes this, so that only taps that
result in an actual change are logged (e.g. trying to change from light mode
to light mode is not logged).

Change-Id: I11946d63514957b06947d4870e591f2c2b03e643