Augment SELinuxMMAC functionality.

* No longer support a package name stanza outside of
  a signature tag. Package names, by themselves, have
  no security associated with them in Android and thus we
  should not be allowing or encouraging this
  type of policy.

* Allow for nested package name stanzas inside
  signature stanzas. There are cases where a finer
  distinction needs to be made among apps signed with
  the same cert. New code allows a different seinfo
  tag to be assigned to the listed package names
  signed by the parent cert. When a determination needs
  to be made concerning seinfo assignments, the inner
  seinfo tag takes precedence over the outer seinfo
  labels which are assigned to just the signature.

* Temp structures are now used to parse new policy files
  until the entire xml file is parsed and deemed correct,
  at which time the temp structures are copied over to the
  permanent class structures. This ensures that any structural
  errors with the policy will not result in partial loads.

* Valid stanzas look like the following with the inner
  package piece being optional.

   <signer signature="">
     <seinfo value=""/>
     <package name="">
       <seinfo value=""/>
     </package>
   <signer>

   <default>
     <seinfo value=""/>
   </default>

Change-Id: Ia204d71211776dcf9b2dcc86ad6d77c4ad39dc25