Send broadcast when package is 'about' to be removed

If a theme package is removed and it is the currently applied
theme, some apps will crash because the theme resources are
no longer available, since they were cleaned up by the package
manager.  The themes provider will receive this broadcast and
take care of switching back to the default theme for components
that are using the package about to be removed.

Change-Id: I968c25e19eb17ec87e7ddcba5ac0fd81745cef20