Change key-set/public-key manifest relationship.

Separate definition of public keys and keysets in the manifest to better
represent their relationship. The 'key-set' tags should have nested additional
'public-key' tags that indicate which of the defined 'public-key' tags are
associated with them.  The first use of a given 'public-key' name should define
its value; subsequent uses may refer to it only by name.  'key-set' and
'public-key' names may not intersect.

Also, change 'keys' tag to 'key-sets' to avoid issues with previous keysets
implementation.

Bug: 6967056
Change-Id: I7534e4a42326e97b67b55509187c0d3c21a2bb32