Remove OmniSwitch preference if removed from system/priv-app

Some users go a bit crazy with removing shit with either Titanium Backup or
other apps and sometimes they remove OmniSwitch. When they do, they're left
with the preference and wonder why it FCs and so report it as a "bug".

This should make it idiot proof (hopefully haha)

Change-Id: I3406709f65bf86f0fc3c6ee8dcbcc9c709fe5a56