Refactor KeySet code.

Eliminate dependency in packagesetting keyset metadata on other packages by
introducing reference counts for KeySets and public keys.  This also allows
keysets to retain their id across reboots by eliminating the need to remove
all keyset data after scanning all packages on boot, which also should
drastically reduce the number of calls to ArraySet.removeAll().

Bug: 19617481

Change-Id: I6cc65f30e431b8e4ebe49047a9219a0d983f2774