Add new <path-permission tag for use by global search.

This adds a new <path-permission> tag you can use inside of a <provide>
to define additional path-based permissions that broaden the global
read and write permissions.  The initial use for this will be global
search, so that a content provider that is protected by permissions
can make a part of itself available to global search under another
permission.  This addresses the issue with global search not being able
to request permissions it would need of providers it doesn't know
about at build time.