Add new "preinstalled" permission flag.

This allows you to specify that a permission can be granted to
any pre-installed system app (not just privileged ones).

And as long as I am doing this, clean up the old "system" permission
flag, renaming it to "privileged" which is what it really is today,
deprecating the old names.  And switch the platform's permission
declarations to use the new name.

Change-Id: Iabf484746af232144786851ec7fe90e3de9dddb2