Themes: Add theme change timestamp to ThemeConfig

Rather than perform two configuration changes when an applied theme
is upadted, this patch adds an additional member to the ThemeConfig
which tracks the time at which the theme is applied.  This allows us
to only perform one configuration change since the new config should
differ from the previous one if the time stamp changes.

The case where this is needed is when a currently applied theme is
updated and needs to be re-applied so that apps pick up the new
resources.

Change-Id: I0036fd6465428bb8e972e294a99839e55203cfaf